A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn

offspring In biology, offspring are the young creation of living organisms, produced either by sexual reproduction, sexual or asexual reproduction. Collective offspring may be known as a brood or progeny. This can refer to a set of simultaneous offspring ...

of a

viviparous In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juve ...

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

that develops from an

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

. Following the embryonic stage, the fetal stage of development takes place.

Prenatal development Prenatal development () involves the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal de ...

is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, in general a fetus is characterized by the presence of all the major body

organs In a multicellular organism, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to a ...

, though they will not yet be fully developed and functional, and some may not yet be situated in their final anatomical location. In human prenatal development, fetal development begins from the ninth week after

fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give ...

(which is the eleventh week of

gestational age In obstetrics, gestational age is a measure of the age of a pregnancy taken from the beginning of the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method, if available. Such metho ...

) and continues until the

birth Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...

of a

newborn In common terminology, a baby is the very young offspring of adult human beings, while infant (from the Latin word ''infans'', meaning 'baby' or 'child') is a formal or specialised synonym. The terms may also be used to refer to Juvenile (orga ...

Etymology

The word ''

fetus A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic development, embryonic stage, the fetal stage of development takes place. Pren ...

'' (plural ''

fetuses A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a ...

'' or rarely, the solecism '' feti''''

Oxford English Dictionary The ''Oxford English Dictionary'' (''OED'') is the principal historical dictionary of the English language, published by Oxford University Press (OUP), a University of Oxford publishing house. The dictionary, which published its first editio ...

'', 2013
''s.v.'' 'fetus'
) comes from

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

'' fētus'' 'offspring, bringing forth, hatching of young'. The Latin plural ''fetūs'' is not used in English; occasionally the plural ''feti'' is used in English by analogy with second-declension Latin nouns. The predominant British, Irish, and

Commonwealth A commonwealth is a traditional English term for a political community founded for the common good. The noun "commonwealth", meaning "public welfare, general good or advantage", dates from the 15th century. Originally a phrase (the common-wealth ...

spelling is ''

foetus A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a ...

'', except in medical usage, where ''fetus'' is preferred. The ''-oe-'' spelling is first attested in 1594 and arose in

Late Latin Late Latin is the scholarly name for the form of Literary Latin of late antiquity.Roberts (1996), p. 537. English dictionary definitions of Late Latin date this period from the 3rd to 6th centuries CE, and continuing into the 7th century in ...

by analogy with classical Latin words like ''amoenus''.

Non-human animals

A fetus is a stage in the

prenatal development Prenatal development () involves the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal de ...

organisms. This stage lies between

embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...

and birth. Many vertebrates have fetal stages, ranging from most mammals to many fish. In addition, some invertebrates bear live young, including some species of

onychophora Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, ''Peripatus''), is a phylum of el ...

and many

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

s. The fetuses of most mammals are situated similarly to the human fetus within their mothers. However, the anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus of a litter-bearing animal is surrounded by placental tissue and is lodged along one of two long uteri instead of the single uterus found in a human female. Development at birth varies considerably among animals, and even among mammals.

Altricial Precocial species in birds and mammals are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. Altricial ...

species are relatively helpless at birth and require considerable parental care and protection. In contrast,

precocial Precocial species in birds and mammals are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. Altricial ...

animals are born with open eyes, have hair or down, have large brains, and are immediately mobile and somewhat able to flee from, or defend themselves against, predators.

Primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...

s are precocial at birth, with the exception of humans. The duration of gestation in placental mammals varies from 18 days in jumping mice to 23 months in

elephant Elephants are the largest living land animals. Three living species are currently recognised: the African bush elephant ('' Loxodonta africana''), the African forest elephant (''L. cyclotis''), and the Asian elephant ('' Elephas maximus ...

s.Sumich, James and Dudley, Gordon
Laboratory and Field Investigations in Marine Life
page 320 (Jones & Bartlett 2008). Generally speaking, fetuses of larger land mammals require longer gestation periods. The benefits of a fetal stage means that young are more developed when they are born. Therefore, they may need less parental care and may be better able to fend for themselves. However, carrying fetuses exerts costs on the mother, who must take on extra food to fuel the growth of her offspring, and whose mobility and comfort may be affected (especially toward the end of the fetal stage). In some instances, the presence of a fetal stage may allow organisms to time the birth of their offspring to a favorable season.

Development in humans

Weeks 9 to 16 (2 to 3.6 months)

In humans, the fetal stage starts nine weeks after fertilization.Klossner, N. Jayne
Introductory Maternity Nursing
(2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth" At this time the fetus is typically about in length from crown to rump, and weighs about 8 grams. The head makes up nearly half of the size of the fetus. Breathing-like movements of the fetus are necessary for the stimulation of lung development, rather than for obtaining oxygen. The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation.
The Columbia Encyclopedia
'' (Sixth Edition). Retrieved 2007-03-05. Uncontrolled movements and twitches occur as

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

s, the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

, and pathways begin to develop.Prechtl, Heinz
"Prenatal and Early Postnatal Development of Human Motor Behavior"
in ''Handbook of brain and behaviour in human development'', Kalverboer and Gramsbergen eds., pp. 415–418 (2001 Kluwer Academic Publishers): "The first movements to occur are sideward bendings of the head. ... At 9–10 weeks postmestrual age complex and generalized movements occur. These are the so-called general movements (Prechtl et al., 1979) and the startles. Both include the whole body, but the general movements are slower and have a complex sequence of involved body parts, while the startle is a quick, phasic movement of all limbs and trunk and neck."

Weeks 17 to 25 (3.6 to 6.6 months)

A woman pregnant for the first time ( nulliparous) typically feels fetal movements at about 21 weeks, whereas a woman who has given birth before will typically feel movements by 20 weeks. By the end of the fifth month, the fetus is about long.

Weeks 26 to 38 (6.6 to 8.6 months)

The amount of body fat rapidly increases. Lungs are not fully mature. Neural connections between the

sensory cortex The sensory cortex can refer sometimes to the primary somatosensory cortex, or it can be used as a term for the primary and secondary cortices of the different senses (two cortices each, on left and right hemisphere): the visual cortex on the occ ...

and

thalamus The thalamus (: thalami; from Greek language, Greek Wikt:θάλαμος, θάλαμος, "chamber") is a large mass of gray matter on the lateral wall of the third ventricle forming the wikt:dorsal, dorsal part of the diencephalon (a division of ...

develop as early as 24 weeks of gestational age, but the first evidence of their function does not occur until around 30 weeks. Bones are fully developed but are still soft and pliable.

Iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

, and

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

become more abundant. Fingernails reach the end of the fingertips. The

lanugo Lanugo is very thin, soft, usually unpigmented hair that is sometimes found on the body of a fetus or newborn. It is the first hair to be produced by the fetal hair follicles, and it usually appears around sixteen weeks of gestation and is abunda ...

, or fine hair, begins to disappear until it is gone except on the upper arms and shoulders. Small breast buds are present in both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 38th week after fertilization. The fetus is considered full-term between weeks 37 and 40 when it is sufficiently developed for life outside the uterus. It may be in length when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until

puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a female, the testicles i ...

.Becher, Julie-Claire. , ''Behind the Medical Headlines'' (Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow October 2004)

Variation in growth

There is much variation in the growth of the human fetus. When the fetal size is less than expected, the condition is known as intrauterine growth restriction also called fetal growth restriction; factors affecting fetal growth can be ''maternal'', ''

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

l'', or ''fetal''.Holden, Chris and MacDonald, Anita.
Nutrition and Child Health
'' (Elsevier 2000). Retrieved 2007-03-04. * Maternal factors include maternal

weight In science and engineering, the weight of an object is a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition. Some sta ...

body mass index Body mass index (BMI) is a value derived from the mass (Mass versus weight, weight) and height of a person. The BMI is defined as the human body weight, body mass divided by the square (algebra), square of the human height, body height, and is ...

, nutritional state,

emotional stress In psychology, stress is a feeling of emotional strain and pressure. Stress is a form of psychological and mental discomfort. Small amounts of stress may be beneficial, as it can improve athletic performance, motivation and reaction to the env ...

, toxin exposure (including

tobacco Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

heroin Heroin, also known as diacetylmorphine and diamorphine among other names, is a morphinan opioid substance synthesized from the Opium, dried latex of the Papaver somniferum, opium poppy; it is mainly used as a recreational drug for its eupho ...

, and other drugs which can also harm the fetus in other ways), and uterine

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...

flow. * Placental factors include size, microstructure (densities and architecture), umbilical blood flow, transporters and binding proteins, nutrient utilization, and nutrient production. * Fetal factors include the fetal genome, nutrient production, and

hormone A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...

output. Also, female fetuses tend to weigh less than males, at full term. Fetal growth is often classified as follows:

small for gestational age Small for gestational age (SGA) newborns are those who are smaller in size than normal for the gestational age. SGA is most commonly defined as a weight below the 10th percentile for the gestational age. SGA predicts susceptibility to hypoglycemi ...

(SGA), appropriate for gestational age (AGA), and large for gestational age (LGA). SGA can result in low birth weight, although premature birth can also result in low birth weight. Low birth weight increases the risk for perinatal mortality (

death Death is the end of life; the irreversible cessation of all biological functions that sustain a living organism. Death eventually and inevitably occurs in all organisms. The remains of a former organism normally begin to decompose sh ...

shortly after birth),

asphyxia Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects all the tissues and organs, some more rapidly than others. There are m ...

hypothermia Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe ...

polycythemia Polycythemia (also known as polycythaemia) is a laboratory finding in which the hematocrit (the volume percentage of red blood cells in the blood) and/or hemoglobin concentration are increased in the blood. Polycythemia is sometimes called erythr ...

hypocalcemia Hypocalcemia is a medical condition characterized by low calcium levels in the blood serum. The normal range of blood calcium is typically between 2.1–2.6 mmol/L (8.8–10.7 mg/dL, 4.3–5.2 mEq/L), while levels less than 2.1 ...

, immune dysfunction, neurologic abnormalities, and other long-term health problems. SGA may be associated with growth delay, or it may instead be associated with absolute stunting of growth.

Viability

Fetal viability Fetal viability is the ability of a human fetus to survive outside the uterus. Viability depends upon factors such as birth weight, gestational age, and the availability of advanced medical care. In low-income countries, more than 90% of extr ...

refers to a point in fetal development at which the fetus may survive outside the womb. The lower limit of viability is approximately months gestational age and is usually later. There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.Moore, Keith and Persaud, T
''The Developing Human: Clinically Oriented Embryology''
p. 103 (Saunders 2003). According to data from 2003 to 2005, survival rates are 20–35% for babies born at 23 weeks of gestation ( months); 50–70% at 24–25 weeks (6 – months); and >90% at 26–27 weeks ( – months) and over.March of Dimes – Neonatal Death
, retrieved September 2, 2009. It is rare for a baby weighing less than to survive. When such

premature Premature may refer to: * ''Premature'' (2014 film), an American comedy film * ''Premature'' (2019 film), an American romantic drama film * '' PREMature'', a 2015 British television drama miniseries See also * Premature aging, of an organism * ...

babies are born, the main causes of mortality are that neither the respiratory system nor the central nervous system are completely differentiated. If given expert postnatal care, some preterm babies weighing less than may survive, and are referred to as ''extremely low birth weight'' or ''immature infants''.

Preterm birth Preterm birth, also known as premature birth, is the Childbirth, birth of a baby at fewer than 37 weeks Gestational age (obstetrics), gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 ...

is the most common cause of infant mortality, causing almost 30 percent of neonatal deaths. At an occurrence rate of 5% to 18% of all deliveries, it is also more common than postmature birth, which occurs in 3% to 12% of pregnancies.

Circulatory system

Before birth

The

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

and

blood vessel Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...

s of the

circulatory system In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart ...

form relatively early during

embryonic development In developmental biology, animal embryonic development, also known as animal embryogenesis, is the developmental stage of an animal embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm, sperm cell (spermat ...

, but continue to grow and develop in complexity in the growing fetus. A functional circulatory system is a biological necessity since mammalian tissues can not grow more than a few cell layers thick without an active blood supply. The prenatal circulation of blood is different from postnatal circulation, mainly because the lungs are not in use. The fetus obtains

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

and nutrients from the mother through the

and the

umbilical cord In Placentalia, placental mammals, the umbilical cord (also called the navel string, birth cord or ''funiculus umbilicalis'') is a conduit between the developing embryo or fetus and the placenta. During prenatal development, the umbilical cord i ...

.Whitaker, Kent (2001)
''Comprehensive Perinatal and Pediatric Respiratory Care''.
Delmar. Retrieved 2007-03-04. Blood from the placenta is carried to the fetus by the

umbilical vein The umbilical vein is a vein present during fetal development that carries oxygenated blood from the placenta into the growing fetus. The umbilical vein provides convenient access to the central circulation of a neonate for restoration of blood vo ...

. About half of this enters the fetal '' ductus venosus'' and is carried to the

inferior vena cava The inferior vena cava is a large vein that carries the deoxygenated blood from the lower and middle body into the right atrium of the heart. It is formed by the joining of the right and the left common iliac veins, usually at the level of the ...

, while the other half enters the

liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...

proper from the inferior border of the liver. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the

portal vein The portal vein or hepatic portal vein (HPV) is a blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver. This blood contains nutrients and toxins extracted from digested contents. Approxima ...

. The blood then moves to the right atrium of the

. In the fetus, there is an opening between the right and left atrium (the '' foramen ovale''), and most of the blood flows from the right into the left atrium, thus bypassing

pulmonary circulation The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lun ...

. The majority of blood flow is into the left ventricle from where it is pumped through the

aorta The aorta ( ; : aortas or aortae) is the main and largest artery in the human body, originating from the Ventricle (heart), left ventricle of the heart, branching upwards immediately after, and extending down to the abdomen, where it splits at ...

into the body. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries and re-enters the placenta, where

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

and other waste products from the fetus are taken up and enter the mother's circulation. Some of the blood from the right atrium does not enter the left atrium, but enters the right ventricle and is pumped into the

pulmonary artery A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and ...

. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ''

ductus arteriosus The ductus arteriosus, also called the ductus Botalli, named after the Italian physiologist Leonardo Botallo, is a blood vessel in the developing fetus connecting the trunk of the pulmonary artery to the proximal descending aorta. It allows mos ...

'', which directs most of this blood away from the lungs (which are not being used for respiration at this point as the fetus is suspended in

amniotic fluid The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products betwee ...

). File:Ultrasound_image_of_a_fetus.jpg,

3D ultrasound 3D ultrasound is a medical ultrasound technique, often used in fetal, cardiac, trans-rectal and intra-vascular applications. 3D ultrasound refers specifically to the volume rendering of ultrasound data. When involving a series of 3D volumes colle ...

of fetus (about months

) File:Sucking his thumb and waving.jpg, Fetus at months File:3dultrasound 20 weeks.jpg, Fetus at 5 months

Postnatal development

With the first breath after birth, the system changes suddenly.

Pulmonary The lungs are the primary organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the backbone on either side of the heart. Their function in the respiratory syste ...

resistance is reduced dramatically, prompting more blood to move into the

pulmonary arteries A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and ...

from the

right atrium The atrium (; : atria) is one of the two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular mitral and tricuspid heart valves. ...

and ventricle of the heart and less to flow through the '' foramen ovale'' into the

left atrium The atrium (; : atria) is one of the two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular mitral and tricuspid heart valves. ...

. The blood from the lungs travels through the

pulmonary vein The pulmonary veins are the veins that transfer Blood#Oxygen transport, oxygenated blood from the lungs to the heart. The largest pulmonary veins are the four ''main pulmonary veins'', two from each lung that drain into the left atrium of the h ...

s to the left atrium, producing an increase in pressure that pushes the '' septum primum'' against the '' septum secundum'', closing the ''foramen ovale'' and completing the separation of the newborn's

into the standard left and right sides. Thereafter, the ''foramen ovale'' is known as the ''fossa ovalis''. The ''

'' normally closes within one or two days of birth, leaving the ''

ligamentum arteriosum The ligamentum arteriosum (arterial ligament), also known as Botallo's ligament, Harvey's ligament, and Botallo's duct, is a small ligament attaching the aorta to the pulmonary artery. It serves no function in adults but is the remnant of the duct ...

'', while the

and '' ductus venosus'' usually closes within two to five days after birth, leaving, respectively, the liver's '' ligamentum teres'' and '' ligamentum venosus''.

Immune system

The

functions as a maternal-fetal barrier against the transmission of

microbe A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...

s. When this is insufficient, mother-to-child transmission of infectious diseases can occur. Maternal IgG antibodies cross the placenta, giving the fetus

passive immunity In immunology, passive immunity is the transfer of active humoral immunity of ready-made antibodies. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta, and it can also be induced arti ...

against those diseases for which the mother has antibodies. This transfer of antibodies in humans begins as early as the fifth month (gestational age) and certainly by the sixth month.

Developmental problems

A developing fetus is highly susceptible to anomalies in its growth and metabolism, increasing the risk of

birth defect A birth defect is an abnormal condition that is present at birth, regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can range from mild to severe. Birth de ...

s. One area of concern is the lifestyle choices made during pregnancy. Diet is especially important in the early stages of development. Studies show that supplementation of the person's diet with

folic acid Folate, also known as vitamin B9 and folacin, is one of the B vitamins. Manufactured folic acid, which is converted into folate by the body, is used as a dietary supplement and in food fortification as it is more stable during processing and ...

reduces the risk of

spina bifida Spina bifida (SB; ; Latin for 'split spine') is a birth defect in which there is incomplete closing of the vertebral column, spine and the meninges, membranes around the spinal cord during embryonic development, early development in pregnancy. T ...

and other

neural tube In the developing chordate (including vertebrates), the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural folds become elevated, ...

defects. Another dietary concern is whether breakfast is eaten. Skipping breakfast could lead to extended periods of lower than normal nutrients in the maternal blood, leading to a higher risk of prematurity, or birth defects. Alcohol consumption may increase the risk of the development of

fetal alcohol syndrome Fetal alcohol spectrum disorders (FASDs) are a group of conditions that can occur in a person who is exposed to alcohol during gestation. FASD affects 1 in 20 Americans, but is highly misdiagnosed and underdiagnosed. The several forms of the ...

, a condition leading to

intellectual disability Intellectual disability (ID), also known as general learning disability (in the United Kingdom), and formerly mental retardation (in the United States), Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010).Archive is a generalized neurodevelopmental ...

in some infants. Smoking during pregnancy may also lead to

miscarriage Miscarriage, also known in medical terms as a spontaneous abortion, is an end to pregnancy resulting in the loss and expulsion of an embryo or fetus from the womb before it can fetal viability, survive independently. Miscarriage before 6 weeks ...

s and low birth weight (. Low birth weight is a concern for medical providers due to the tendency of these infants, described as "''premature'' by weight", to have a higher risk of secondary medical problems.

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

s are known to have possible adverse effects on the development of the fetus, and the risks need to be weighed against the benefits. Congenital disorders are acquired before birth. Infants with certain

congenital heart defect A congenital heart defect (CHD), also known as a congenital heart anomaly, congenital cardiovascular malformation, and congenital heart disease, is a defect in the structure of the heart or great vessels that is present at birth. A congenital h ...

s can survive only as long as the ductus remains open: in such cases the closure of the ductus can be delayed by the administration of

prostaglandin Prostaglandins (PG) are a group of physiology, physiologically active lipid compounds called eicosanoids that have diverse hormone-like effects in animals. Prostaglandins have been found in almost every Tissue (biology), tissue in humans and ot ...

s to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of

patent ductus arteriosus Patent ductus arteriosus (PDA) is a medical condition in which the ''ductus arteriosus'' fails to close after childbirth, birth: this allows a portion of oxygenated blood from the left heart to flow back to the lungs from the aorta, which has a h ...

, where the ductus does not properly close, drugs that inhibit prostaglandin synthesis can be used to encourage its closure, so that surgery can be avoided. Other heart birth defects include

ventricular septal defect A ventricular septal defect (VSD) is a defect in the ventricular septum, the wall dividing the left and right ventricles of the heart. It's a common heart problem present at birth ( congenital heart defect). The extent of the opening may vary ...

, pulmonary atresia, and

tetralogy of Fallot Tetralogy of Fallot (TOF), formerly known as Steno-Fallot tetralogy, is a congenital heart defect characterized by four specific cardiac defects. Classically, the four defects are: * Pulmonary stenosis, which is narrowing of the exit from the r ...

. An abdominal pregnancy can result in the death of the fetus and where this is rarely not resolved it can lead to its formation into a lithopedion.

Fetal pain

The existence and implications of fetal pain are debated politically and academically. According to the conclusions of a review published in 2005, "Evidence regarding the capacity for fetal pain is limited but indicates that fetal perception of pain is unlikely before the third trimester." Two authors of the study published in JAMA did not report their abortion-related activities, which pro-life groups called a conflict of interest; the editor of JAMA responded that JAMA probably would have mentioned those activities if they had been disclosed, but still would have published the study. See Denise Grady
"Study Authors Didn't Report Abortion Ties"
, ''New York Times'' (2005-08-26)."Study: Fetus feels no pain until third trimester"
NBC News However, developmental neurobiologists argue that the establishment of thalamocortical connections (at about months) is an essential event with regard to fetal perception of pain.Johnson, Martin and Everitt, Barry.
Essential reproduction
' (Blackwell 2000): "The multidimensionality of pain perception, involving sensory, emotional, and cognitive factors may in itself be the basis of conscious, painful experience, but it will remain difficult to attribute this to a fetus at any particular developmental age." Retrieved 2007-02-21. Nevertheless, the perception of pain involves sensory, emotional and cognitive factors and it is "impossible to know" when pain is experienced, even if it is known when thalamocortical connections are established. Some authors argue that fetal pain is possible from the second half of

pregnancy Pregnancy is the time during which one or more offspring gestation, gestates inside a woman's uterus. A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Conception (biology), Conception usually occurs ...

. Evidence suggests that the perception of pain in the fetus occurs well before late gestation. Whether a fetus has the ability to feel pain and

suffering Suffering, or pain in a broad sense, may be an experience of unpleasantness or aversion, possibly associated with the perception of harm or threat of harm in an individual. Suffering is the basic element that makes up the negative valence (psyc ...

is part of the

abortion debate The abortion debate is a longstanding and contentious discourse that touches on the moral, legal, medical, and religious aspects of induced abortion. In English-speaking countries, the debate has two major sides, commonly referred to as the "pro- ...

. In the United States, for example, anti-abortion advocates have proposed legislation that would require providers of abortions to inform pregnant women that their fetuses may feel pain during the procedure and that would require each person to accept or decline

anesthesia Anesthesia (American English) or anaesthesia (British English) is a state of controlled, temporary loss of sensation or awareness that is induced for medical or veterinary purposes. It may include some or all of analgesia (relief from or prev ...

for the fetus.

Legal and social issues

Abortion of a human pregnancy is legal and/or tolerated in most countries, although with gestational time limits that normally prohibit late-term abortions.Anika Rahman, Laura Katzive and Stanley K. Henshaw.
A Global Review of Laws on Induced Abortion, 1985–1997
", ''International Family Planning Perspectives'' Volume 24, Number 2 (June 1998).

References

External links

Prenatal Image Gallery Index
at the Endowment for Human Development website, featuring numerous motion pictures of human fetal movement.
''In the Womb''
(National Geographic video).

MedlinePlus Medical Encyclopedia {{Authority control Animal developmental biology Embryology Fertility