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A fetus (also spelled foetus, fœtus, faetus or fætus, see below) is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth. The plural is fetuses.

In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.[1] [2]


Etymology and spelling variations

The word fetus is from the Latin fetus, meaning offspring, bringing forth, hatching of young.[3] It has Indo-European roots related to sucking or suckling, from the Aryan prefix bheu-, meaning "To come into being".[4]

Fœtus or foetus is an "incorrectly written" English spelling that has however been in use since at least 1594.[4] 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 Saint Isidore of Seville, in AD 620.[5] This spelling is nevertheless the most common in most English-speaking countries (except in professional literature). The original spelling based on the correct Latin etymology fetus however persists in Canada and the United States. In addition, fetus is now also the standard English spelling throughout the world in medical journals and other technical usage.[6] The spelling "faetus" was used historically.[7]

The only correct English plural is fetuses. The pseudo-Latin plural feti is sometimes seen in English, but this is based on insufficient knowledge of Latin. (In Latin, fētus is fourth declension, and its plural is fētūs.)



9 weeks of gestation: condition at start of fetal stage

Artist's depiction of fetus 11 weeks after fertilization. The crown-rump length is 1.25 inches.[8]

The fetal stage commences at the beginning of the 9th week.[1] At the start of the fetal stage, the fetus is typically about 30 mm (1.2 inches) in length from crown to rump, and weighs about 8 grams.[1] The head makes up nearly half of the fetus' size.[9] Breathing-like movement of the fetus is necessary for 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]

Fetuses are not capable of feeling pain at the beginning of the fetal stage, and will not be able to feel pain until the third trimester.[13] At this point in development, uncontrolled movements and twitches occur as muscles, the brain and pathways begin to develop.[14]

16 to 25 weeks after fertilization

A woman pregnant for the first time (i.e. a primiparous woman) typically feels fetal movements at about 21 weeks, whereas a woman who has already given birth at least two times (i.e. a multiparous woman) will typically feel movements by 20 weeks.[15] By the end of the fifth month, the fetus is about 20 cm (8 inches).

26 to 40 weeks of gestation

Artist's depiction of fetus at 40 weeks after fertilization, about 20 inches (51 cm) head to toe.

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 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 40th week. The fetus is considered full-term between weeks 35 and 40,[16] which means that the fetus is considered sufficiently developed for life outside the uterus.[17] 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.[18][19]

Variation in growth

There is much variation in the growth of the 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.[20]

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.[20]

Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA).[21] 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.


Stages in prenatal development, showing viability and point of 50% chance of survival at bottom. Weeks and months numbered by gestation.

The lower limit of viability is approximately five months gestational age, and usually later.[22]

Human fetus, age unknown

There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. [23] 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.[24] It is rare for a baby weighing less than 500 gm to survive.[23]

When such babies are born, the main causes of perinatal mortality is that the respiratory system and the central nervous system are not completely differentiated.[23] If given expert postnatal care, some fetuses weighing less than 500 gm may survive, being are referred to as extremely low birth weight or immature infants.[23] 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."[13][26] However, there may be an emerging consensus among developmental neurobiologists that the establishment of thalamocortical connections" (at about 26 weeks) is a critical event with regard to fetal perception of pain.[27] Nevertheless, because pain can involve sensory, emotional and cognitive factors, it is "impossible to know" when painful experiences may become possible, even if it is known when thalamocortical connections are established.[27]

Whether a fetus has the ability to feel pain and to suffer is part of the abortion debate.[28] [29] 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.[30]

Circulatory system

Diagram of the human fetal circulatory system.

The circulatory system of a human fetus works differently from that of born humans, mainly because the lungs are not in use: the fetus obtains oxygen and nutrients from the woman through the placenta and the umbilical cord.[31]

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.[31]

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).[31]

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 from the adult circulatory system

Remnants of the fetal circulation can be found in adults:[32][33]

Fetal Adult
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 than adults (adults use adult hemoglobin). 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.

Developmental problems

Congenital anomalies are anomalies that 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.

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.[34] 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 the woman eats breakfast. 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.[35] 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.

Legal issues

Abortion of a fetus is legal in many countries such as Australia, India, Canada, most European countries, and the United States, despite the disapproval by certain groups of the act of destorying what they view as a human life. Many of those countries that allow abortion during the fetal stage have gestational time limits, so that late-term abortions are not normally allowed.[36] There is almost universal agreement that the strongest justification for abortion is to save the life of the mother.[37]

See also


  1. ^ a b c Klossner, N. Jayne Introductory Maternity Nursing (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth"
  2. ^ The American Pregnancy Association
  3. ^ Harper, Douglas. (2001). Online Etymology Dictionary. Retrieved 2007-01-20.
  4. ^ a b Fetus, Oxford English Dictionary, http://dictionary.oed.com/cgi/entry/50087237 
  5. ^ Aronson, Jeff (26 July 1997). "When I use a word...:Oe no!". British Medical Journal 315 (7102). http://bmj.bmjjournals.com/cgi/content/full/315/7102/0/h. 
  6. ^ New Oxford Dictionary of English.
  7. ^ American Dictionary of the English Language. Noah Webster. (1828).
  8. ^ Marc H. Bornstein, Michael E. Lamb. Developmental Science: An Advanced Textbook, page 227 (2005): “At 8 weeks, fetuses measure 3.18 cm from crown to rump (1.25 inches).”
  9. ^ MedlinePlus
  10. ^ Institute of Medicine of the National Academies, Preterm Birth: Causes, Consequences, and Prevention (2006), page 317. Retrieved 2008-03-12
  11. ^ The Columbia Encyclopedia (Sixth Edition). Retrieved 2007-03-05.
  12. ^ Greenfield, Marjorie. “Dr. Spock.com". Retrieved 2007-01-20.
  13. ^ a b Lee, Susan; Ralston, HJ; Drey, EA; Partridge, JC; Rosen, MA (August 24/31, 2005). "Fetal Pain A Systematic Multidisciplinary Review of the Evidence". The Journal of the American Medical Association (the American Medical Association) 294 (8): 947. doi:10.1001/jama.294.8.947. PMID 16118385. http://jama.ama-assn.org/cgi/content/full/294/8/947. Retrieved 2008-02-14.  (see Fetal Pain section)
  14. ^ 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."
  15. ^ Levene, Malcolm et al. Essentials of Neonatal Medicine (Blackwell 2000), p. 8. Retrieved 2007-03-04.
  16. ^ Your Pregnancy: 36 Weeks BabyCenter.com Retrieved June 1, 2007.
  17. ^ "full-term" defined by Memidex/WordNet.
  18. ^ Stanley, Fiona et al. "Cerebral Palsies: Epidemiology and Causal Pathways", page 48 (2000 Cambridge University Press): "Motor competance at birth is limited in the human neonate. The voluntary control of movement develops and matures during a prolonged period up to puberty...."
  19. ^ Becher, Julie-Claire. "Insights into Early Fetal Development", Behind the Medical Headlines (Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow October 2004)
  20. ^ a b Holden, Chris and MacDonald, Anita. Nutrition and Child Health (Elsevier 2000). Retrieved 2007-03-04.
  21. ^ Queenan, John. Management of High-Risk Pregnancy (Blackwell 1999). Retrieved 2007-03-04.
  22. ^ Halamek, Louis. "Prenatal Consultation at the Limits of Viability", 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."
  23. ^ a b c d Moore, Keith and Persaud, T. The Developing Human: Clinically Oriented Embryology, p. 103 (Saunders 2003).
  24. ^ March of Dimes --> Neonatal Death Retrieved on September 2, 2009
  25. ^ March of Dimes --> Neonatal Death Retrieved on September 2, 2009
  26. ^ "Study: Fetus feels no pain until third trimester" MSNBC
  27. ^ a b 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.
  28. ^ White, R. Frank. "Are We Overlooking Fetal Pain and Suffering During Abortion?", American Society of Anesthesiologists Newsletter (October 2001). Retrieved 2007-03-10.
  29. ^ David, Barry & and Goldberg, Barth. "Recovering Damages for Fetal Pain and Suffering", Illinois Bar Journal (December 2002). Retrieved 2007-03-10.
  30. ^ Weisman, Jonathan. "House to Consider Abortion Anesthesia Bill", Washington Post 2006-12-05. Retrieved 2007-02-06.
  31. ^ a b c Whitaker, Kent. Comprehensive Perinatal and Pediatric Respiratory Care (Delmar 2001). Retrieved 2007-03-04.
  32. ^ Dudek, Ronald and Fix, James. Board Review Series Embryology (Lippincott 2004). Retrieved 2007-03-04.
  33. ^ University of Michigan Medical School, Fetal Circulation and Changes at Birth. Retrieved 2007-03-04.
  34. ^ Dalby, JT. (1978).Environmental effects on prenatal development Journal of Pediatric Psychology, 3, 105-109.
  35. ^ Streissguth, Ann Pytkowicz (1997). Fetal alcohol syndrome: a guide for families and communities. Baltimore, MD: Paul H Brookes Pub. ISBN 1-55766-283-5. 
  36. ^ Anika Rahman, Laura Katzive and Stanley K. Henshaw. A Global Review of Laws on Induced Abortion, 1985-1997, International Family Planning Perspectives (Volume 24, Number 2, June 1998).
  37. ^ Wennberg, Robert. Life in the Balance, p. 139 (Eerdmans Publishing 1985).

External links

Preceded by
Stages of human development
Succeeded by


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