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A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after
fertilization 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. Pro ...
(or eleventh week gestational age) and continues until birth. 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.


Etymology

The word '' fetus'' (plural '' fetuses'' or '' feti'') 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 a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
'' fētus'' ("offspring", "bringing forth", "hatching of young") and the Greek "φυτώ" to plant. The word "fetus" was used by
Ovid Pūblius Ovidius Nāsō (; 20 March 43 BC – 17/18 AD), known in English as Ovid ( ), was a Augustan literature (ancient Rome), Roman poet who lived during the reign of Augustus. He was a contemporary of the older Virgil and Horace, with whom ...
in
Metamorphoses The ''Metamorphoses'' ( la, Metamorphōsēs, from grc, μεταμορφώσεις: "Transformations") is a Latin narrative poem from 8 CE by the Roman poet Ovid. It is considered his '' magnum opus''. The poem chronicles the history of the ...
, book 1, line 104. The predominant British, Irish, and Commonwealth spelling is '' foetus'', 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 form of Literary Latin of late antiquity.Roberts (1996), p. 537. English dictionary definitions of Late Latin date this period from the , and continuing into the 7th century in t ...
, in which the distinction between the vowel sounds ''-oe-'' and ''-e-'' had been lost. This spelling is the most common in most 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 '' faetus'' was also used historically.


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 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 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.
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 ( 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 and
thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direct ...
develop as early as 24 weeks' gestational age, but the first evidence of their function does not occur until around 30 weeks, when minimal
consciousness Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
, dreaming, and the ability to feel pain emerges. Bones are fully developed, but are still soft and pliable.
Iron Iron () is a chemical element with symbol Fe (from la, ferrum) 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, right in ...
,
calcium Calcium is a chemical element with the 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 ...
, and
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
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 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 girl, the testes in a ...
.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 Intrauterine growth restriction (IUGR), or fetal growth restriction, refers to poor growth of a fetus while in the womb during pregnancy. IUGR is defined by clinical features of malnutrition and evidence of reduced growth regardless of an infant's ...
also called fetal growth restriction; factors affecting fetal growth can be ''maternal'', ''
placenta The placenta 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 the physically separate mate ...
l'', 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, 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, heroin, 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 in the cir ...
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 Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...
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 Prenatal development () includes 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 devel ...
(AGA), and large for gestational age (LGA). SGA can result in
low birth weight Low birth weight (LBW) is defined by the World Health Organization as a birth weight of an infant of or less, regardless of gestational age. Infants born with LBW have added health risks which require close management, often in a neonatal int ...
, although premature birth can also result in low birth weight. Low birth weight increases risk for perinatal mortality (
death Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...
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 primarily the tissues and organs. There are many circumstances that can ...
,
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 e ...
, 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.


Viability

Fetal viability Fetal viability is the ability of a human fetus to survive outside the uterus. Medical viability is generally considered to be between 23 and 24 weeks gestational age. Viability depends upon factors such as birth weight, gestational age, and the ...
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 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 Preterm birth, also known as premature birth, is the birth of a baby at fewer than 37 weeks gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 weeks, very early preterm birth is betwee ...
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 in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
and
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide awa ...
s of the
circulatory system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
, form relatively early during 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 Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
and nutrients from the mother through the
placenta The placenta 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 the physically separate mate ...
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 The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
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 The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
. 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 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 Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
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. 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 are not being used for respiration at this point as the fetus is suspended in amniotic fluid). 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 collec ...
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. Pulmonary resistance is reduced dramatically, prompting more blood to move into the pulmonary arteries from the right atrium and ventricle of the heart and less to flow through the '' 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 The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
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 The ligamentum arteriosum (arterial ligament), also known as the Ligament of Botallo or Harvey's ligament, is a small ligament attaching the aorta to the pulmonary artery. It serves no function in adults but is the remnant of the ductus arteriosus ...
'', while the umbilical vein and '' ductus venosus'' usually closes within two to five days after birth, leaving, respectively, the liver's '' ligamentum teres'' and ''
ligamentum venosus The ligamentum venosum, also known as Arantius' ligament, is the fibrous remnant of the ductus venosus of the fetal circulation. Usually, it is attached to the left branch of the portal vein within the porta hepatis. It may be continuous with the r ...
''.


Immune system

The
placenta The placenta 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 the physically separate mate ...
functions as a
maternal-fetal barrier The placenta 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 the physically separate matern ...
against the transmission of microbes. When this is insufficient, mother-to-child transmission of infectious diseases can occur. Maternal
IgG antibodies Immunoglobulin G (Ig G) is a type of antibody. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in blood circulation. IgG molecules are created and released by plasma B cells. Each IgG a ...
cross the placenta, giving the fetus
passive immunity 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 artificially, when ...
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, also known as a congenital disorder, 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 ca ...
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 a ...
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 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 during pregnancy may also lead to miscarriages and
low birth weight Low birth weight (LBW) is defined by the World Health Organization as a birth weight of an infant of or less, regardless of gestational age. Infants born with LBW have added health risks which require close management, often in a neonatal int ...
(. 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, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
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 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 ''Patent ductus arteriosus'' (PDA) is a medical condition in which the ''ductus arteriosus'' fails to close after birth: this allows a portion of oxygenated blood from the left heart to flow back to the lungs by flowing from the aorta, which has ...
, 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 Pulmonary atresia is a congenital malformation of the pulmonary valve in which the valve orifice fails to develop. The valve is completely closed thereby obstructing the outflow of blood from the heart to the lungs. The pulmonary valve is located ...
, 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 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 develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ...
. Evidence suggests that the perception of pain in the fetus occurs well before late gestation Glover V. The fetus may feel pain from 20 weeks. ''Conscience''. 2004-2005 Winter;25(3):35-7 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 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 of a human pregnancy is legal and/or tolerated in most countries, although with gestational time limits that normally prohibit late-term abortions.


Other animals

A fetus is a stage in the 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
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
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 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, predators.
Primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter includin ...
s are precocial at birth, with the exception of humans.Lewin, Roger
Human Evolution
page 78 (Blackwell 2004).
The duration of gestation in placental mammals varies from 18 days in jumping mice to 23 months in
elephant Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantida ...
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.


See also


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