Fetus

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This article is about the stage of prenatal development. For other uses, see Fetus (disambiguation).
"Foetus" redirects here. For the band, see Foetus (band). For the film, see Foetus (film).
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A fetus or foetus (

newborn. 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 fētus ("offspring", "bringing forth", "hatching of young")[2][3][4] and the Greek "φυτώ" to plant. The word "fetus" was used by Ovid in Metamorphoses, book 1, line 104.[5]

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, 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.[6] The spelling faetus was also used historically.[7]

Development in humans

Further information: Prenatal development

Weeks 9 to 16 (2 to 3.6 months)

A human fetus, attached to placenta, at three months gestational age

In humans, the fetal stage starts nine weeks after fertilization.[8] At the start of the fetal stage, the fetus is typically about 30 millimetres (1+1⁄4 in) in length from crown-rump, and weighs about 8 grams.[8] The head makes up nearly half of the size of the fetus.[9] Breathing-like movements of the fetus are necessary for the stimulation of lung development, rather than for obtaining oxygen.[10] The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation.[11][12]

At this point in development, uncontrolled movements and twitches occur as muscles, the brain, and pathways begin to develop.[13]

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.[14] By the end of the fifth month, the fetus is about 20 cm (8 in) 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 develop as early as 24 weeks of gestational age, but the first evidence of their function does not occur until around 30 weeks.[citation needed] 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 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.[15][16] It may be 48 to 53 cm (19 to 21 in) in length when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until puberty.[17][18]

Variation in growth

Further information: Birth weight and Environmental toxicants and fetal development

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, placental, or fetal.[19]

Maternal factors include maternal

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 fetal genome, nutrient production, and hormone output. Also, female fetuses tend to weigh less than males, at full term.[19]

Fetal growth is often classified as follows:

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

Main article: Fetal viability
by gestation
.

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 5+3⁄4 months gestational age and is usually later.[21]

There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.

weeks of gestation (5+3⁄4 months); 50–70% at 24–25 weeks (6 – 6+1⁄4 months); and >90% at 26–27 weeks (6+1⁄2 – 6+3⁄4 months) and over.[23] It is rare for a baby weighing less than 500 g (1 lb 2 oz) to survive.[22]

When such premature 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 500 g (1 lb 2 oz) may survive, and are referred to as extremely low birth weight or immature infants.[22]

Preterm birth is the most common cause of infant mortality, causing almost 30 percent of neonatal deaths.[23] At an occurrence rate of 5% to 18% of all deliveries,[24] it is also more common than postmature birth, which occurs in 3% to 12% of pregnancies.[25]

Circulatory system

Main article: Fetal circulation

Before birth

fetal circulatory system

The

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.[26]

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 mother's circulation.[26]

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).[26]

  • 3D ultrasound of 80-millimetre (3 in) fetus (about 3+1⁄2 months gestational age)
    gestational age
    )
  • Fetus at 4+1⁄4 months
    Fetus at 4+1⁄4 months
  • Fetus at 5 months
    Fetus at 5 months

Postnatal development

Main article: Adaptation to extrauterine life

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

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

ligamentum venosus
.

Immune system

The

mother-to-child transmission
of infectious diseases can occur.

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 fifth month (gestational age) and certainly by the sixth month.[27]

Developmental problems

Further information: Environmental toxicants and fetal development and Birth defect

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

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 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.[29] Smoking during pregnancy may also lead to miscarriages and low birth weight
(2,500 grams (5 pounds 8 ounces). 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.[30][31]

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

Main article: Prenatal perception

The existence and implications of

thalamocortical connections (at about 6+1⁄2 months) is an essential event with regard to fetal perception of pain.[34][page needed] 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.[34] Some authors argue that fetal pain is possible from the second half of pregnancy. Evidence suggests that the perception of pain in the fetus occurs well before late gestation.[35]

Whether a fetus has the ability to feel pain and suffering is part of the abortion debate.[36][37][38] 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.[39]

Legal and social issues

late-term abortions.[40]

Other animals

Further information: Evolution of mammals
Fourteen phases of elephant development before birth

A fetus is a stage in the

embryogenesis and birth.[1] Many vertebrates have fetal stages, ranging from most mammals to many fish. In addition, some invertebrates bear live young, including some species of onychophora[41] and many arthropods
.

The fetuses of most mammals are situated similarly to the human fetus within their mothers.

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.

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. Primates are precocial at birth, with the exception of humans.[43]

The duration of gestation in

jumping mice to 23 months in elephants.[44] Generally speaking, fetuses of larger land mammals require longer gestation periods.[44]

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.[41]

See also

References

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  4. ^ "Charlton T. Lewis, An Elementary Latin Dictionary, fētus". Archived from the original on 2017-01-04. Retrieved 2015-09-24.
  5. ^ Miller, Frank Justus (1951) [1916]. Ovid Metamorphoses. Vol. 1. Cambridge, Massachusetts and London: Harvard University Press and William Heinemann Ltd. p. 8. ark:/13960/t9n30qx2z.
  6. ^ New Oxford Dictionary of English.
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  8. ^ a b Klossner, N. Jayne, Introductory Maternity Nursing (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth"
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  10. ^ Institute of Medicine of the National Academies, Preterm Birth: Causes, Consequences, and Prevention Archived 2011-06-07 at the Wayback Machine (2006), page 317. Retrieved 2008-03-12
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  13. ^ 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."
  14. ^ Levene, Malcolm et al. Essentials of Neonatal Medicine Archived 2023-04-08 at the Wayback Machine (Blackwell 2000), p. 8. Retrieved 2007-03-04.
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  16. ^ "Giving Birth Before Your Due Date: Do All 40 Weeks Matter?". Parents. Archived from the original on 2022-11-02. Retrieved 2022-11-01.
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  22. ^ a b c Moore, Keith and Persaud, T. The Developing Human: Clinically Oriented Embryology, p. 103 (Saunders 2003).
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