A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring of an animal that develops from an
embryo An embryo is the early stage of development of a multicellular organism. In general, in organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodi ...
. Following
embryonic development ''Embryonic'' is the twelfth studio album by experimental rock Experimental rock, also called avant-rock, is a subgenre of rock music that pushes the boundaries of common composition and performance technique or which experiments with the basic el ...
the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after
fertilisation Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proc ...
(or eleventh week gestational age) and continues until
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 fe ...
. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final anatomical location.


The word ''
fetus A fetus American and British English spelling differences#ae and oe, or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring of an animal that develops from an embryo. Following embryonic development the fetal stage of dev ...

'' (plural ''
fetuses A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring of an animal that develops from an embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, feta ...

'' or ''
'') is related to the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the ...
'' fētus'' ("offspring", "bringing forth", "hatching of young") and the Greek "φυτώ" to plant. The predominant British, Irish, and Commonwealth spelling is ''
'', which has been in use since at least 1594. The spelling with ''-oe-'' arose in
Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area ar ...
, in which the distinction between the vowel sounds ''-oe-'' and ''-e-'' had been lost. This spelling is the most common in most
Commonwealth nations
Commonwealth nations
, except in the medical literature, where ''fetus'' is used. The more classical spelling ''fetus'' is used in Canada and the United States. In addition, ''fetus'' is now the standard English spelling throughout the world in medical journals. The spelling ''
'' was also used historically.


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 the start of the fetal stage, the fetus is typically about in length from crown-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 , 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.
The genitalia of the fetus starts to form and placenta becomes fully functional during week 9. At this point in development, uncontrolled movements and twitches occur as muscles, the brain, 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 (Gravidity and parity, 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 20 cm (8 inches) long.

Weeks 26 to 38 (6.6 to 8.6 months)

The amount of body fat rapidly increases. Lungs are not fully mature. thalamus, Thalamic brain connections, which mediate sensory input, form. Bones are fully developed, but are still soft and pliable. Iron, calcium, and phosphorus become more abundant. Fingernails reach the end of the fingertips. The lanugo, or fine hair, begins to disappear, until it is gone except on the upper arms and shoulders. Small breast buds are present on 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 36 and 40, when it is sufficiently developed for life outside the uterus. It may be 48 to 53 cm (19 to 21 inches) in length, when born. Control of movement is limited at birth, and purposeful voluntary movements develop all the way until puberty.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 fetal size is less than expected, the condition is known as intrauterine growth restriction (IUGR) also called fetal growth restriction (FGR); factors affecting fetal growth can be ''maternal'', ''placental'', or ''fetal''.Holden, Chris and MacDonald, Anita.
Nutrition and Child Health
' (Elsevier 2000). Retrieved 2007-03-04.
Maternal factors include maternal weight, body mass index, nutritional state, emotional Stress (medicine), stress, Environmental toxicants and fetal development, toxin exposure (including tobacco, alcohol (drug), alcohol, heroin, and other drugs which can also harm the fetus in other ways), and uterus, uterine blood flow. Placental factors include size, microstructure (densities and architecture), umbilical cord, umbilical blood flow, transporters and binding proteins, nutrient utilization and nutrient production. Fetal factors include the fetus genome, nutrient production, and hormone output. Also, female fetuses tend to weigh less than males, at full term. Fetal growth is often classified as follows: small for gestational age (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 risk for perinatal mortality (death shortly after birth), asphyxia, hypothermia, polycythemia, hypocalcemia, 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.


Fetal viability 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 2003–05, survival rates are 20–35% for babies born at 23 gestational age, 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 Preterm birth, premature babies are born, the main causes of infant, mortality are that the respiratory system and the central nervous system are not 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 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 Postterm pregnancy, postmature birth, which occurs in 3% to 12% of pregnancies.

Circulatory system

Before birth

The heart and blood vessels of the circulatory system, form relatively early during human embryogenesis, embryonic development, 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 and nutrients from the mother through the placenta and the umbilical cord.Whitaker, Kent.
Comprehensive Perinatal and Pediatric Respiratory Care
' (Delmar 2001). Retrieved 2007-03-04.
Blood from the placenta is carried to the fetus by the umbilical vein. About half of this enters the fetal ''ductus venosus'' and is carried to the inferior vena cava, while the other half enters the liver 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 blood then moves to the right atrium of the heart. In the fetus, there is an opening between the right and left atrium (the ''foramen ovale (heart), foramen ovale''), and most of the blood flows from the right into the left atrium, thus bypassing pulmonary circulation. The majority of blood flow is into the left ventricle from where it is pumped through the aorta 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 and other waste products from the fetus are taken up and enter the woman'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. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ''ductus arteriosus'', which directs most of this blood away from the lungs (which aren't being used for respiration at this point as the fetus is suspended in amniotic fluid). File:Ultrasound_image_of_a_fetus.jpg, 3D ultrasound of fetus (about months gestational age) 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. Lung, Pulmonary resistance is reduced dramatically, prompting more blood to move into the Pulmonary artery, pulmonary arteries from the right atrium and Right ventricle, ventricle of the heart and less to flow through the ''Foramen ovale (heart), foramen ovale'' into the left atrium. The blood from the lungs travels through the pulmonary veins 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 circulatory system into the standard left and right sides. Thereafter, the ''foramen ovale'' is known as the ''fossa ovalis''. The ''ductus arteriosus'' normally closes within one or two days of birth, leaving the ''ligamentum arteriosum'', while the umbilical vein and ''ductus venosus'' usually closes within two to five days after birth, leaving, respectively, the liver's ''Round ligament of the uterus, ligamentum teres'' and ''ligamentum venosus''.

Immune system

The placenta functions as a Placenta#Immunity, maternal-fetal barrier against the transmission of microbes. When this is insufficient, mother-to-child transmission of infectious diseases can occur. Maternal Immunoglobulin G, IgG antibodies cross the placenta, giving the fetus passive immunity 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 defects. 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 reduces the risk of spina bifida and other neural tube 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 Preterm birth, prematurity, or birth defects. Alcohol consumption may increase the risk of the development of fetal alcohol syndrome, a condition leading to intellectual disability in some infants. Smoking and pregnancy, Smoking during pregnancy may also lead to miscarriages and low birth weight (2500 grams, 5.5 lb). 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-rays 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 defects 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 prostaglandins to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of patent ductus arteriosus, 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, pulmonary atresia, and tetralogy of Fallot. 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

Fetal pain, its existence and its implications 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 Neurobiology, neurobiologists argue that the establishment of Human thalamus, 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 Glover V. The fetus may feel pain from 20 weeks. ''Conscience''. 2004-2005 Winter;25(3):35-7 argue that fetal pain is possible from the second half of pregnancy: “The available scientific evidence makes it possible, even probable, that fetal pain perception occurs well before late gestation” wrote KJS Anand in the journal of the International Association for the Study of Pain, IASP.http://www.iasp-pain.org/AM/AMTemplate.cfm?Section=HOME&TEMPLATE=/CM/ContentDisplay.cfm&CONTENTID=15390&SECTION=HOME Whether a fetus has the ability to feel pain and suffering is part of the abortion debate. In the United States, for example, Anti-abortion movements, 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 for the fetus.

Legal and social issues

Abortion law, Abortion of a human pregnancy is legal and/or tolerated in most countries, although with gestational time limits that normally prohibit Late-term abortion#Legal restrictions on later abortion, late-term abortions.

Other animals

A fetus is a stage in the prenatal development (biology), prenatal development of viviparous organisms. This stage lies between embryogenesis 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 and many arthropods. 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 (animal), litter-bearing animals compared to humans: each fetus of a litter-bearing animal is surrounded by placental Biological tissue, 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 species are relatively helpless at birth and require considerable parental care and protection. In contrast, precocial 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, predation, predators. Primates are precocial at birth, with the exception of humans.Lewin, Roger
Human Evolution
page 78 (Blackwell 2004).
The duration of gestation in Placentalia, placental mammals varies from 18 days in jumping mice to 23 months in elephants.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. Image:Walembryo 2.jpg, left, Fetal stage of a porpoise 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.

See also


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