A fetus or foetus (/ˈfiːtəs/; pl.: fetuses, foetuses, rarely feti or foeti) is the unborn mammalianoffspring that develops from an embryo.[1] Following the embryonic stage, the fetal stage of development takes place. Prenatal development 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, 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 (which is the eleventh week of gestational age) and continues until the birth of a newborn.
Etymology
The word fetus (plural fetuses or rarely feti[2]) comes from Latinfētus 'offspring, bringing forth, hatching of young'.[3][4][5] The Latin plural fetūs is not used in English; occasionally the plural feti is used in English by analogy with second-declension Latin nouns ending in -us.[2]
The predominant British, Irish, and Commonwealth spelling is foetus, except in medical usage, where fetus is preferred. The -oe- spelling is first attested in 1594 and arose in Late Latin by analogy with classical Latin words like amoenus.[6]
A human fetus, attached to placenta, at three months gestational age
In humans, the fetal stage starts nine weeks after fertilization.[7] At this time the fetus is typically about 30 millimetres (1+1⁄4 in) in length from crown to rump, and weighs about 8 grams.[7] The head makes up nearly half of the size of the fetus.[8] Breathing-like movements of the fetus are necessary for the stimulation of lung development, rather than for obtaining oxygen.[9] The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation.[10][11] Uncontrolled movements and twitches occur as muscles, the brain, and pathways begin to develop.[12]
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.[13] 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.[14][15] 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.[16][17]
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.[18]
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.[18]
Stages in prenatal development, showing viability and point of 50% chance of survival at bottom. Weeks and months numbered 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.[20]
There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.[21] According to data from 2003 to 2005, survival rates are 20–35% for babies born at 23 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.[22] It is rare for a baby weighing less than 500 g (1 lb 2 oz) to survive.[21]
When such premature 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 500 g (1 lb 2 oz) may survive, and are referred to as extremely low birth weight or immature infants.[21]
Preterm birth is the most common cause of infant mortality, causing almost 30 percent of neonatal deaths.[22] At an occurrence rate of 5% to 18% of all deliveries,[23] it is also more common than postmature birth, which occurs in 3% to 12% of pregnancies.[24]
The heart and blood vessels of 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 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.[25]
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.[25]
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).[25]
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 into the standard left and right sides. Thereafter, the foramen ovale is known as the fossa ovalis.
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.[26]
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.[27] 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 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.[28]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.[29][30]
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.
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."[31][32] However, developmental neurobiologists argue that the establishment of thalamocortical connections (at about 6+1⁄2 months) is an essential event with regard to fetal perception of pain.[33][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.[33] 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.[34]
Whether a fetus has the ability to feel pain and suffering is part of the abortion debate.[35][36][37] 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.[38]
Legal and social issues
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Fourteen phases of elephant development before birth
A fetus is a stage in the prenatal development of viviparous organisms. This stage lies between 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[40] and many arthropods.
The fetuses of most mammals are situated similarly to the human fetus within their mothers.[41] 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. Primates are precocial at birth, with the exception of humans.[42]
The duration of gestation in placental mammals varies from 18 days in jumping mice to 23 months in elephants.[43] Generally speaking, fetuses of larger land mammals require longer gestation periods.[43]
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.[40]
^ abKlossner, N. Jayne, Introductory Maternity Nursing (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth"
^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."
^Stanley, Fiona et al. "Cerebral Palsies: Epidemiology and Causal Pathways", page 48 (2000 Cambridge University Press): "Motor competence at birth is limited in the human neonate. The voluntary control of movement develops and matures during a prolonged period up to puberty...."
^Becher, Julie-Claire. "Insights into Early Fetal Development". Archived from the original on 2013-06-01., Behind the Medical Headlines (Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow October 2004)
^Halamek, Louis. "Prenatal Consultation at the Limits of ViabilityArchived 2009-06-08 at the Wayback Machine", NeoReviews, Vol.4 No.6 (2003): "most neonatologists would agree that survival of infants younger than approximately 22 to 23 weeks' estimated gestational age [i.e. 20 to 21 weeks' estimated fertilization age] is universally dismal and that resuscitative efforts should not be undertaken when a neonate is born at this point in pregnancy."
^Page 202 of Pillitteri, Adele (2009). Maternal and Child Health Nursing: Care of the Childbearing and Childrearing Family. Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN978-1-58255-999-5.
^Dalby, JT (1978). "Environmental effects on prenatal development". Journal of Pediatric Psychology. 3 (3): 105–109. doi:10.1093/jpepsy/3.3.105.
^De Santis, M; Cesari, E; Nobili, E; Straface, G; Cavaliere, AF; Caruso, A (September 2007). "Radiation effects on development". Birth Defects Research Part C: Embryo Today: Reviews. 81 (3): 177–82. doi:10.1002/bdrc.20099. PMID17963274.
^Lee, Susan; Ralston, HJ; Drey, EA; Partridge, JC; Rosen, MA (August 24–31, 2005). "Fetal Pain A Systematic Multidisciplinary Review of the Evidence". Journal of the American Medical Association. 294 (8): 947–54. doi:10.1001/jama.294.8.947. PMID16118385. 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"Archived 2009-04-25 at the Wayback Machine, New York Times (2005-08-26).
^ abJohnson, 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.
^"Fetal pain?". International Association for the Study of Pain. June 2006. Archived from the original on 2013-07-01.
^"Unborn babies can feel pain". Minnesota Citizens Concerned for Life. Archived from the original on 2016-07-19. The neural pathways are present for pain to be experienced quite early by unborn babies," explains Steven Calvin, M.D., perinatologist, chair of the Program in Human Rights Medicine, University of Minnesota, where he teaches obstetrics.
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