Fetus

A fetus /ˈfiːtəs/, also spelled foetus, fœtus, faetus, or fætus, is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth. It is also defined as the unborn young of a vertebrate, after developing to its basic form.^[1]

In humans, the fetal stage of prenatal development may be defined as beginning at the 11th week in gestational age, which is the 9th week after fertilization.^[2][3] In biological terms, however, prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. The use of the term "fetus" generally implies that a mammalian embryo has developed to the point of being recognizable as belonging to its own species, and this is usually taken to be the 9th week after fertilization. A fetus is also characterized by the presence of all the major body organs, though they will not yet be fully developed and functional, and may not all be situated in their final anatomical location.

Etymology

The word fetus (plural fetuses) is from the Latin fētus (“offspring”, “bringing forth”, “hatching of young”).^[4][5]

Fœtus or foetus is the British, Irish and Commonwealth spelling, which has been in use since at least 1594.^[6] It arose as a hypercorrection based on an incorrect etymology (i.e. due to insufficient knowledge of Latin) that may have originated with an error by Isidore of Seville, in AD 620.^[7][8] This spelling is the most common in most Commonwealth nations, except in the medical literature, where fetus is used. The etymologically accurate original 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.^[9] The spelling faetus was used historically.^[10]

Development

Weeks 9 to 16

In humans, the fetal stage commences at the beginning of the ninth week.^[2] At the start of the fetal stage, the fetus is typically about 30 millimetres (1.2 in) in length from crown to rump, and weighs about 8 grams.^[2] The head makes up nearly half of the fetus' size.^[11] Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.^[12] The heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation.^[13][14]

At this point in development, uncontrolled movements and twitches occur as muscles, the brain, and pathways begin to develop.^[15]

Weeks 17 to 25

A woman pregnant for the first time (primiparous), typically feels fetal movements at about 21 weeks, whereas a woman who has given birth at least twice (multiparous), will typically feel movements by 20 weeks.^[16] By the end of the fifth month, the fetus is about 20 cm (8 inches) long.

Weeks 26 to 38

The amount of body fat rapidly increases. Lungs are not fully mature. 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 35 and 38, when it is sufficiently developed for life outside the uterus.^[17][18] 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.^[19][20]

Variation in growth

There is much variation in the growth of the human fetus. When fetal size is less than expected, that condition is known as intrauterine growth restriction (IUGR) also called fetal growth restriction (FGR); factors affecting fetal growth can be maternal, placental, or fetal.^[21]

Maternal factors include maternal weight, body mass index, nutritional state, emotional stress, toxin exposure (including tobacco, alcohol, heroin, and other drugs which can also harm the fetus in other ways), and uterine blood 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 fetus genome, nutrient production, and hormone output. Also, female fetuses tend to weigh less than males, at full term.^[21]

Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA).^[22] 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.

Viability

Viability refers to a point in fetal development at which the fetus may survive outside the womb. The lower limit of viability is approximately five months gestational age, and usually later.^[23]

There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.^[24] According to data years 2003-2005, 20 to 35 percent of babies born at 23 weeks of gestation survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive.^[25] It is rare for a baby weighing less than 500 gm to survive.^[24]

When such premature babies are born, the main causes of perinatal mortality are that the respiratory system and the central nervous system are not completely differentiated.^[24] If given expert postnatal care, some fetuses weighing less than 500 gm may survive, and are referred to as extremely low birth weight or immature infants.^[24] Preterm birth is the most common cause of perinatal mortality, causing almost 30 percent of neonatal deaths.^[25]

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."^[26][27] However, developmental neurobiologists argue that the establishment of thalamocortical connections (at about 26 weeks), is an essential event with regard to fetal perception of pain.^[28] 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.^[28] Some authors,^[29] however, 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 IASP.^[30]

Whether a fetus has the ability to feel pain and to suffer is part of the abortion debate.^[31][32] For example, in the USA legislation has been proposed by pro-life advocates that abortion providers should be required to tell a woman that the fetus may feel pain during the abortion procedure, and require her to accept or decline anesthesia for the fetus.^[33]

Circulatory system

The heart and blood vessels which form the circulatory system, 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 than the 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.^[34]

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), 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.^[34]

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).^[34]

Postnatal development

With the first breath after birth, the system changes suddenly. The pulmonary resistance is dramatically reduced ("pulmo" is from the Latin for "lung"). More blood moves from the right atrium to the right ventricle and into the pulmonary arteries, and less flows through the foramen ovale to the left atrium. The blood from the lungs travels through the pulmonary veins to the left atrium, increasing the pressure there. The decreased right atrial pressure and the increased left atrial pressure pushes the septum primum against the septum secundum, closing the foramen ovale, which now becomes the fossa ovalis. This completes the separation of the circulatory system into two halves, the left and the right.

The ductus arteriosus normally closes off within one or two days of birth, leaving behind the ligamentum arteriosum. The umbilical vein and the ductus venosus closes off within two to five days after birth, leaving behind the ligamentum teres and the ligamentum venosus of the liver respectively.

Differences between fetal and postnatal

Remnants of the fetal circulation can be found in the adult.^[35][36]

Fetal	Developed
foramen ovale	fossa ovalis
ductus arteriosus	ligamentum arteriosum
extra-hepatic portion of the fetal left umbilical vein	ligamentum teres hepatis (the "round ligament of the liver").
intra-hepatic portion of the fetal left umbilical vein (the ductus venosus)	ligamentum venosum
proximal portions of the fetal left and right umbilical arteries	umbilical branches of the internal iliac arteries
distal portions of the fetal left and right umbilical arteries	medial umbilical ligaments (urachus)

In addition to differences in circulation, the developing fetus also employs a different type of oxygen transport molecule in its hemoglobin, from that when it is born and breathing its own oxygen. Fetal hemoglobin enhances the fetus' ability to draw oxygen from the placenta. Its dissociation curve to oxygen is shifted to the left, meaning that it will take up oxygen at a lower concentration than adult hemoglobin will. This enables fetal hemoglobin to absorb oxygen from adult hemoglobin in the placenta, which has a lower pressure of oxygen than at the lungs. In the human infant, until about six months old, the hemoglobin molecule is made up of 2 alpha and 2 gamma chains (2α2γ). The gamma chains are gradually replaced by beta chains until the molecule becomes that of hemoglobin A with its two chains of alpha and two chains of beta (2α2β).

• 

  3D ultrasound of 3-inch (76 mm) fetus (about 14 weeks gestational age)

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  Fetus at 17 weeks

• 

  Fetus at 20 weeks

Immune system

The placenta functions as a maternal-fetal barrier against the transmission of microbes, and when this is insufficient, mother-to-child transmission of infectious diseases can occur.

Also, maternal 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 20th week of gestational age, and certainly by the 24th week.^[37]

Human 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 pregnant woman's lifestyle choices made during pregnancy.^[38] Diet is especially important in the early stages of development. Studies show that supplementation of the woman'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 woman's blood, leading to a higher risk of prematurity, or other birth defects in the fetus. During this time alcohol consumption may increase the risk of the development of fetal alcohol syndrome, a condition leading to mental retardation in some infants.^[39] Smoking during pregnancy may also lead to reduced birth weight. Low birth weight is defined as 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. Some research shows that fetal ultrasounds (including Doppler, 3D/4D ultrasound, and 2D ultrasound) can have negative effect on birth weight and neurodevelopment.^[40]

Congenital anomalies are acquired before birth. Infants with certain congenital anomalies of the heart 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.

Legal issues

Abortion of a human pregnancy is legal and/or tolerated due to facets of the physician-patient relationship in many countries such as Australia, India, Canada, most European countries, and the United States. Many of those countries that allow abortion during the fetal stage have gestational time limits, so that late-term abortions are not normally allowed.^[41]

See also

• Abortion
• Fetal position
• Fetal rights
• Fetoscopy
• Neural development
• Potential person
• Pregnancy
• Prenatal development
• Superfetation
• Women's rights

References

External links

Wikimedia Commons has media related to Fetus.

• "Prenatal Image Gallery Index" from The Endowment for Human Development (providing numerous motion pictures of human fetal movement that can be viewed online).
• "In the Womb," video from National Geographic.

Preceded by Embryo	Stages of human development Fetus	Succeeded by Infancy

v t e Developmental biology > Human embryogenesis (development of embryo) and development of fetus (TE E2.0) First three weeks Week 1 Fertilization Oocyte activation Zygote Cleavage Morula Blastula Blastomere Blastocyst Inner cell mass Week 2 (Bilaminar) Hypoblast Epiblast Week 3 (Trilaminar) Germ layers Archenteron/Primitive streak Primitive pit Primitive knot/Blastopore Primitive groove Gastrula/Gastrulation Regional specification Embryonic disc Ectoderm Surface ectoderm Neuroectoderm Somatopleuric mesenchyme Neurulation Neural crest Endoderm Splanchnopleuric mesenchyme Mesoderm Chorda- Paraxial (Somite/Somitomere) Intermediate Lateral plate Intraembryonic coelom Splanchnopleuric mesenchyme/Somatopleuric mesenchyme M: OBS phys/devp/memb mthr/fetu/infc, epon proc, drug (2A/G2C)