View source for Biology:Fetus

{{Short description|Prenatal organism between the embryonic state and birth}}
{{Human growth and development}}

A '''fetus''' or '''foetus''' ({{IPAc-en|ˈ|f|iː|t|ə|s}}; {{plural form}}: '''fetuses''', '''feti''', '''foetuses''', or '''foeti''') is the unborn offspring that develops from an animal [[Biology:Embryo|embryo]].<ref name=":0">{{Citation|last1=Ghosh|first1=Shampa|title=Fetus|date=2017|encyclopedia=Encyclopedia of Animal Cognition and Behavior|pages=1–5|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-319-47829-6_62-1|isbn=9783319478296|last2=Raghunath|first2=Manchala|last3=Sinha|first3=Jitendra Kumar}}</ref> Following [[Biology:Embryonic development|embryonic development]], the fetal stage of development takes place. In human [[Biology:Prenatal development|prenatal development]], fetal development begins from the ninth week after fertilization (or eleventh week gestational age) and continues until [[Medicine:Childbirth|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 [[Biology:Anatomy|anatomical location]].

== Etymology ==
The word ''fetus'' (plural ''fetuses'' or ''feti'') is related to the [[Social:Latin|Latin]] ''fētus'' ("offspring", "bringing forth", "hatching of young")<ref>O.E.D.2nd Ed.2005</ref><ref>Harper, Douglas. (2001). ''[http://etymonline.com/ Online Etymology Dictionary] {{webarchive|url=https://web.archive.org/web/20130420044417/http://etymonline.com/ |date=2013-04-20 }}''. Retrieved 2007-01-20.</ref><ref>{{Cite web |url = https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0060%3Aentry%3Dfetus2 |title = Charlton T. Lewis, An Elementary Latin Dictionary, fētus |access-date = 2015-09-24 |url-status = live |archive-url = https://web.archive.org/web/20170104185503/http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0060%3Aentry%3Dfetus2 |archive-date = 2017-01-04 }}</ref> and the Greek "φυτώ" to plant. The word "fetus" was used by Ovid in ''Metamorphoses'', book 1, line 104.<ref>{{Cite book|last=Miller|first=Frank Justus|title=Ovid Metamorphoses|publisher=Harvard University Press and William Heinemann Ltd|year=1951|volume=1|location=Cambridge, Massachusetts and London|pages=8|id=ark:/13960/t9n30qx2z|orig-year=1916}}</ref>

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 [[Organization:Commonwealth of Nations|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.<ref>''New Oxford Dictionary of English''.</ref> The spelling ''faetus'' was also used historically.<ref>''American Dictionary of the English Language'', Noah Webster, 1828.</ref>

== Development in humans ==
{{Further|Biology:Prenatal development}}

===Weeks 9 to 16 (2 to 3.6 months)===
[[File:Fetus 3 months.jpg|thumb|right|220px|A human fetus, attached to placenta, at three months gestational age]]

In humans, the fetal stage starts nine weeks after fertilization.<ref name="nursing">Klossner, N. Jayne, [https://books.google.com/books?id=B47OVg25g-QC&pg=PA103 Introductory Maternity Nursing] (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth"</ref> At the start of the fetal stage, the fetus is typically about {{convert|30|mm|in|frac=4}} in length from [[Biology:Crown-rump length|crown-rump]], and weighs about 8&nbsp;grams.<ref name="nursing" /> The head makes up nearly half of the size of the fetus.<ref name="itrfps">{{cite web|url=https://www.nlm.nih.gov/medlineplus/ency/article/002398.htm|title=Fetal development: MedlinePlus Medical Encyclopedia|website=www.nlm.nih.gov|url-status=live|archive-url=https://web.archive.org/web/20111027150119/http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm|archive-date=2011-10-27}}</ref> Breathing-like movements of the fetus are necessary for the stimulation of [[Medicine:Lung#Development|lung development]], rather than for obtaining oxygen.<ref>Institute of Medicine of the National Academies, ''[http://books.nap.edu/openbook.php?record_id=11622&page=261 Preterm Birth: Causes, Consequences, and Prevention] {{webarchive|url=https://web.archive.org/web/20110607061030/http://books.nap.edu/openbook.php?record_id=11622&page=261|date=2011-06-07}}'' (2006), page 317. Retrieved 2008-03-12</ref> The heart, hands, feet, brain, and other organs are present, but are only at the beginning of development and have minimal operation.<ref name="Columbia">''[http://www.bartleby.com/65/fe/fetus.html The Columbia Encyclopedia] {{webarchive|url=https://web.archive.org/web/20071012162642/http://bartleby.com/65/fe/fetus.html |date=2007-10-12 }}'' (Sixth Edition). Retrieved 2007-03-05.</ref><ref>Greenfield, Marjorie. "[http://www.drspock.com/article/0,1510,9851,00.html Dr. Spock.com] {{webarchive|url=https://web.archive.org/web/20070122182805/http://www.drspock.com/article/0%2C1510%2C9851%2C00.html |date=2007-01-22 }}". Retrieved 2007-01-20.</ref>

At this point in development, uncontrolled movements and twitches occur as muscles, the brain, and pathways begin to develop.<!--"twitches reference found on pages 233, 472, 726 in Prechtl--><ref name="Prechtl">Prechtl, Heinz. [https://books.google.com/books?id=FzyPozUyKPkC&pg=RA1-PA416 "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.&nbsp;... 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."</ref>

=== Weeks 17 to 25 (3.6 to 6.6 months)===
A woman pregnant for the first time ([[Medicine:Gravidity and parity|nulliparous]]) typically feels [[Biology:Fetal movement|fetal movement]]s at about 21 weeks, whereas a woman who has given birth before will typically feel movements by 20 weeks.<ref>Levene, Malcolm et al. ''[https://books.google.com/books?id=FHXtDkLzOHEC&pg=RA2-PA8 Essentials of Neonatal Medicine] {{Webarchive|url=https://web.archive.org/web/20230408064509/https://books.google.com/books?id=FHXtDkLzOHEC&pg=RA2-PA8 |date=2023-04-08 }}'' (Blackwell 2000), p. 8. Retrieved 2007-03-04.</ref> By the end of the fifth month, the fetus is about {{convert|20|cm|in|0|abbr=on}} 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 [[Biology:Sensory cortex|sensory cortex]] and [[Biology:Thalamus|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|date=November 2022}} Bones are fully developed but are still soft and pliable. [[Chemistry:Iron|Iron]], [[Chemistry:Calcium|calcium]], and [[Chemistry:Phosphorus|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 [[Medicine:Adaptation to extrauterine life|life outside the uterus]].<ref>{{cite web|url=https://www.nhs.uk/pregnancy/week-by-week/28-to-40-plus/37-weeks/#:~:text=At%2037%20weeks%2C%20your%20pregnancy,in%20the%20next%20few%20weeks.|publisher=NHS.UK|title=You and your baby at 37 weeks pregnant|date=8 December 2020|access-date=2022-11-01|archive-date=2022-11-02|archive-url=https://web.archive.org/web/20221102014312/https://www.nhs.uk/pregnancy/week-by-week/28-to-40-plus/37-weeks/#:~:text=At%2037%20weeks%2C%20your%20pregnancy,in%20the%20next%20few%20weeks.|url-status=live}}</ref><ref>{{cite web|url=https://www.parents.com/pregnancy/giving-birth/why-it-pays-to-wait-for-a-full-term-baby-birth/|publisher=Parents|title=Giving Birth Before Your Due Date: Do All 40 Weeks Matter?|access-date=2022-11-01|archive-date=2022-11-02|archive-url=https://web.archive.org/web/20221102014312/https://www.parents.com/pregnancy/giving-birth/why-it-pays-to-wait-for-a-full-term-baby-birth/|url-status=live}}</ref> It may be {{convert|48|to|53|cm|in|0|abbr=on}} in length when born. Control of movement is limited at birth, and purposeful voluntary movements continue to develop until puberty.<ref>Stanley, Fiona et al. [https://books.google.com/books?id=aJwvMYMAfmYC&pg=PA48 "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...."</ref><ref name="Becher">Becher, Julie-Claire. {{cite web|url = http://pregnancyarchive.com/articles/insights-into-early-fetal-development |title = Insights into Early Fetal Development |url-status = dead |archive-url = https://web.archive.org/web/20130601182944/http://pregnancyarchive.com/articles/insights-into-early-fetal-development/ |archive-date = 2013-06-01}}, ''Behind the Medical Headlines'' (Royal College of Physicians of Edinburgh and Royal College of Physicians and Surgeons of Glasgow October 2004) <!-- archived article --></ref>

===Variation in growth===
{{Further|Medicine:Birth weight|Biology: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 [[Medicine:Intrauterine growth restriction|intrauterine growth restriction]] also called fetal growth restriction; factors affecting fetal growth can be ''maternal'', ''[[Biology:Placenta|placenta]]l'', or ''fetal''.<ref name="Holden">Holden, Chris and MacDonald, Anita. ''[https://www.abebooks.com/9780702024214/Nutrition-Child-Health-Chris-Holden-070202421X/plp Nutrition and Child Health] {{Webarchive|url=https://web.archive.org/web/20200731092010/https://www.abebooks.com/9780702024214/Nutrition-Child-Health-Chris-Holden-070202421X/plp |date=2020-07-31 }}'' (Elsevier 2000). Retrieved 2007-03-04.</ref>

'''Maternal''' factors include maternal [[Physics:Weight|weight]], [[Body mass index|body mass index]], nutritional state, emotional stress, [[Biology:Environmental toxicants and fetal development|toxin exposure]] (including [[Biology:Tobacco|tobacco]], [[Chemistry:Alcohol (drug)|alcohol]], [[Chemistry:Heroin|heroin]], and other drugs which can also harm the fetus in other ways), and [[Biology:Uterus|uterine]] [[Biology:Blood|blood]] flow.

'''Placental''' factors include size, microstructure (densities and architecture), [[Biology:Umbilical cord|umbilical]] blood flow, transporters and binding proteins, nutrient utilization, and nutrient production.

'''Fetal''' factors include the fetal genome, nutrient production, and [[Biology:Hormone|hormone]] output. Also, female fetuses tend to weigh less than males, at full term.<ref name="Holden"/>

Fetal growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA).<ref>Queenan, John. ''[https://books.google.com/books?id=sfp-OzoU6X4C&pg=RA2-PA6 Management of High-Risk Pregnancy] {{Webarchive|url=https://web.archive.org/web/20230424003357/https://books.google.com/books?id=sfp-OzoU6X4C&pg=RA2-PA6 |date=2023-04-24 }}'' (Blackwell 1999). Retrieved 2007-03-04.</ref> SGA can result in [[Medicine:Low birth weight|low birth weight]], although premature birth can also result in low birth weight. Low birth weight increases the risk for perinatal mortality ([[Biology:Death|death]] shortly after birth), asphyxia, [[Physics:Hypothermia|hypothermia]], polycythemia, [[Medicine:Hypocalcemia|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 ==
{{wide image|Prenatal development table.svg|1200px|Stages in [[Biology:Prenatal development|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 {{frac|5|3|4}} months gestational age and is usually later.<ref>Halamek, Louis. "[http://neoreviews.aappublications.org/cgi/content/extract/4/6/e153 Prenatal Consultation at the Limits of Viability] {{webarchive|url=https://web.archive.org/web/20090608065459/http://neoreviews.aappublications.org/cgi/content/extract/4/6/e153 |date=2009-06-08 }}", ''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."</ref>

There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.<ref name="developinghuman">Moore, Keith and Persaud, T. [https://books.google.com/books?id=dbRpAAAAMAAJ&q=%22Prematurity+is+one+of+the+most+common+causes+of+morbidity%22 ''The Developing Human: Clinically Oriented Embryology''], p. 103 (Saunders 2003).</ref> According to data from 2003 to 2005, survival rates are 20–35% for babies born at 23 weeks of gestation ({{frac|5|3|4}} months); 50–70% at 24–25 weeks (6 – {{frac|6|1|4}} months); and >90% at 26–27 weeks ({{frac|6|1|2}} – {{frac|6|3|4}} months) and over.<ref name="marchofdimes">[http://www.marchofdimes.org/loss/neonatal-death.aspx March of Dimes - Neonatal Death] {{webarchive|url=https://web.archive.org/web/20141024114718/http://www.marchofdimes.org/loss/neonatal-death.aspx |date=2014-10-24 }}, retrieved September 2, 2009.</ref> It is rare for a baby weighing less than {{convert|500|g|lboz|abbr=on}} to survive.<ref name="developinghuman"/>

When such [[Medicine:Preterm birth|premature]] babies are born, the main causes of mortality are that the respiratory system and the central nervous system are not completely differentiated.<!--<ref name="developinghuman"/>--> If given expert postnatal care, some preterm babies weighing less than {{convert|500|g|lboz|abbr=on}} may survive, and are referred to as ''extremely low birth weight'' or ''immature infants''.<ref name="developinghuman"/>

[[Medicine:Preterm birth|Preterm birth]] is the most common cause of infant mortality, causing almost 30 percent of neonatal deaths.<ref name="marchofdimes"/> At an occurrence rate of 5% to 18% of all deliveries,<ref name="WHO2014">{{cite web |url =https://www.who.int/mediacentre/factsheets/fs363/en/ |title = Preterm birth Fact sheet N°363 |author = World Health Organization |work = who.int |date = November 2014 |access-date = 6 Mar 2015 |url-status = live |archive-url = https://web.archive.org/web/20150307050438/http://www.who.int/mediacentre/factsheets/fs363/en/ |archive-date = 7 March 2015 |df = dmy-all }}</ref> it is also more common than postmature birth, which occurs in 3% to 12% of pregnancies.<ref>{{cite book |last1 = Buck |first1 = Germaine M. |last2 = Platt |first2 = Robert W. |title = Reproductive and perinatal epidemiology |date = 2011 |publisher = Oxford University Press |location = Oxford |isbn = 9780199857746 |page = 163 |url = https://books.google.com/books?id=by1lwSpfruQC&pg=PA163 |url-status = live |archive-url = https://web.archive.org/web/20160815200142/https://books.google.ca/books?id=by1lwSpfruQC&pg=PA163 |archive-date = 2016-08-15 }}</ref>

== Circulatory system ==

===Before birth===
[[File:Fetal circulation.png|thumb|right|220px|Diagram of the human fetal circulatory system]]
The [[Biology:Heart|heart]] and [[Biology:Blood vessel|blood vessel]]s of the [[Biology:Circulatory system|circulatory system]] form relatively early during [[Biology:Human embryogenesis|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 [[Chemistry:Oxygen|oxygen]] and nutrients from the mother through the [[Biology:Placenta|placenta]] and the [[Biology:Umbilical cord|umbilical cord]].<ref name="Whitaker">Whitaker, Kent (2001). [https://web.archive.org/web/20150912194122/https://books.google.com/books?id=R3WK8XyAHYgC&pg=PA18 ''Comprehensive Perinatal and Pediatric Respiratory Care''.] Delmar. Retrieved 2007-03-04.</ref>

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 [[Biology:Liver|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 [[Biology:Heart|heart]]. In the fetus, there is an opening between the right and left atrium (the ''[[Medicine:Foramen ovale (heart)|foramen ovale]]''), and most of the blood flows from the right into the left atrium, thus bypassing [[Biology:Pulmonary circulation|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 [[Chemistry:Carbon dioxide|carbon dioxide]] and other waste products from the fetus are taken up and enter the mother's circulation.<ref name="Whitaker"/>

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 [[Biology:Amniotic fluid|amniotic fluid]]).<ref name="Whitaker"/>

<gallery class="center">
File:Ultrasound_image_of_a_fetus.jpg|[[3D ultrasound|3D ultrasound]] of {{convert|3|in|mm|-1|adj=on|order=flip}} fetus (about {{frac|3|1|2}} months gestational age)
File:Sucking his thumb and waving.jpg|Fetus at {{frac|4|1|4}} months
File:3dultrasound 20 weeks.jpg|Fetus at 5 months
</gallery>

===Postnatal development===
{{main|Medicine:Adaptation to extrauterine life}}
With the first breath after birth, the system changes suddenly. [[Medicine:Lung|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 ''[[Medicine:Foramen ovale (heart)|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 [[Biology:Circulatory system|circulatory system]] 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'', 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''.

== Immune system ==

The [[Biology:Placenta|placenta]] functions as a [[Biology:Placenta#Immunity|maternal-fetal barrier]] against the transmission of microbes. When this is insufficient, mother-to-child transmission of infectious diseases can occur.

Maternal [[Biology:Immunoglobulin G|IgG antibodies]] cross the placenta, giving the fetus [[Biology:Passive immunity|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.<ref>[https://books.google.com/books?id=apeLf0mPx1QC&pg=PA202 Page 202] of {{cite book |author = Pillitteri, Adele |title = Maternal and Child Health Nursing: Care of the Childbearing and Childrearing Family |publisher = Lippincott Williams & Wilkins |location = Hagerstwon, MD |year = 2009 |isbn = 978-1-58255-999-5 }}</ref>

== Developmental problems ==
{{Further|Biology:Environmental toxicants and fetal development|Medicine:Birth defect}}

A developing fetus is highly susceptible to anomalies in its growth and metabolism, increasing the risk of [[Medicine:Birth defect|birth defect]]s. One area of concern is the lifestyle choices made during pregnancy.<ref>{{cite journal |last1 = Dalby |first1 = JT |year = 1978 |title = Environmental effects on prenatal development |journal = Journal of Pediatric Psychology |volume = 3 |issue =  3|pages = 105–109 |doi = 10.1093/jpepsy/3.3.105 }}</ref> 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 [[Medicine:Spina bifida|spina bifida]] and other [[Biology:Neural tube|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 [[Medicine:Preterm birth|prematurity]], or birth defects.

Alcohol consumption may increase the risk of the development of fetal alcohol syndrome, a condition leading to [[Medicine:Intellectual disability|intellectual disability]] in some infants.<ref>{{cite book |author = Streissguth, Ann Pytkowicz |title = Fetal alcohol syndrome: a guide for families and communities |publisher = Paul H Brookes Pub |location = Baltimore, MD |year = 1997 |isbn = 978-1-55766-283-5 |url-access = registration |url = https://archive.org/details/fetalalcoholsynd00stre }}</ref> Smoking during pregnancy may also lead to [[Medicine:Miscarriage|miscarriage]]s and [[Medicine:Low birth weight|low birth weight]] ({{convert|2500|g|lboz|abbr=off}}. 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.

[[Physics:X-ray|X-ray]]s are known to have possible adverse effects on the development of the fetus, and the risks need to be weighed against the benefits.<ref>O'Reilly, Deirdre. "[https://www.nlm.nih.gov/medlineplus/ency/article/002398.htm Fetal development] {{webarchive|url=https://web.archive.org/web/20111027150119/http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm |date=2011-10-27 }}". ''[[Social:MedlinePlus|MedlinePlus Medical Encyclopedia]]'' (2007-10-19). Retrieved 2018-08-26.</ref><ref name="De Santis">{{cite journal |last1=De Santis |first1=M |last2=Cesari |first2=E |last3=Nobili |first3=E |last4=Straface |first4=G |last5=Cavaliere |first5=AF |last6=Caruso |first6=A |title=Radiation effects on development. |journal=Birth Defects Research Part C: Embryo Today: Reviews |date=September 2007 |volume=81 |issue=3 |pages=177–82 |doi=10.1002/bdrc.20099 |pmid=17963274}}</ref>

Congenital disorders are acquired before birth. Infants with certain [[Medicine:Congenital heart defect|congenital heart defect]]s 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,  [[Medicine:Pulmonary atresia|pulmonary atresia]], and [[Medicine:Tetralogy of Fallot|tetralogy of Fallot]].

An [[Medicine:Abdominal pregnancy|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 [[Biology:Lithopedion|lithopedion]].

== Fetal pain ==
{{main|Biology:Prenatal perception}}

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."<ref name="JAMA">{{cite journal |last1 = Lee |first1 = Susan |title = Fetal Pain A Systematic Multidisciplinary Review of the Evidence |journal = Journal of the American Medical Association |volume = 294 |issue = 8 |date = August 24–31, 2005 |pmid = 16118385 |doi = 10.1001/jama.294.8.947 |pages = 947–54 |last2 = Ralston |first2 = HJ |last3 = Drey |first3 = EA |last4 = Partridge |first4 = JC |last5 = Rosen |first5 = MA |doi-access =  }} 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, [https://www.nytimes.com/2005/08/26/health/26pain.html "Study Authors Didn't Report Abortion Ties"] {{webarchive|url=https://web.archive.org/web/20090425131833/http://www.nytimes.com/2005/08/26/health/26pain.html |date=2009-04-25 }}, ''New York Times'' (2005-08-26).</ref><ref name="sskqke">[http://www.nbcnews.com/id/9053416 "Study: Fetus feels no pain until third trimester"] {{Webarchive|url=https://web.archive.org/web/20191028220841/http://www.nbcnews.com/id/9053416 |date=2019-10-28 }}  NBC News</ref> However, developmental neurobiologists argue that the establishment of thalamocortical connections (at about {{frac|6|1|2}} months) is an essential event with regard to fetal perception of pain.<ref name="Johnson">Johnson, Martin and Everitt, Barry. ''[https://books.google.com/books?id=MzZRuSQ5UeEC&pg=PA215 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.</ref>{{Page needed|date=May 2022}} 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.<ref name="Johnson"/> Some authors argue that fetal pain is possible from the second half of [[Medicine:Pregnancy|pregnancy]]. Evidence suggests that the perception of pain in the fetus occurs well before late gestation.<ref name="test">Glover V. The fetus may feel pain from 20 weeks. ''Conscience''. 2004-2005 Winter;25(3):35-7</ref><ref name="Anand">{{cite web |url=http://www.iasp-pain.org/AM/AMTemplate.cfm?Section=HOME&TEMPLATE=/CM/ContentDisplay.cfm&CONTENTID=15390&SECTION=HOME |title=Fetal pain?  |website=www.iasp-pain.org |archive-url=https://web.archive.org/web/20130701122731/http://www.iasp-pain.org/AM/AMTemplate.cfm?Section=HOME&TEMPLATE=%2FCM%2FContentDisplay.cfm&CONTENTID=15390&SECTION=HOME |archive-date=2013-07-01}}</ref>

Whether a fetus has the ability to feel pain and [[Social:Suffering|suffering]] is part of the abortion debate.<ref>White, R. Frank. "New research has discovered that unborn babies can feel pain. "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." [http://www.mccl.org/unborn-babies-can-feel-pain.html Unborn babies can feel pain] {{webarchive|url=https://web.archive.org/web/20160719130222/http://www.mccl.org/unborn-babies-can-feel-pain.html|date=2016-07-19}}", [http://www.asahq.org/Newsletters/2001/10_01/white.htm Are We Overlooking Fetal Pain and Suffering During Abortion?], ''American Society of Anesthesiologists Newsletter'' (October 2001). Retrieved 2007-03-10.</ref><ref>David, Barry and Goldberg, Barth. "[http://www.illinoisbar.org/IBJ/dec02lj/dectoc.htm Recovering Damages for Fetal Pain and Suffering] {{webarchive|url=https://web.archive.org/web/20070928035813/http://www.illinoisbar.org/IBJ/dec02lj/dectoc.htm |date=2007-09-28 }}", ''Illinois Bar Journal'' (December 2002). Retrieved 2007-03-10.</ref> In the United States, for example, [[Social:Anti-abortion movements|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 [[Medicine:Anesthesia|anesthesia]] for the fetus.<ref>Weisman, Jonathan. "[https://www.washingtonpost.com/wp-dyn/content/article/2006/12/04/AR2006120401089.html House to Consider Abortion Anesthesia Bill] {{webarchive|url=https://web.archive.org/web/20081028163732/http://www.washingtonpost.com/wp-dyn/content/article/2006/12/04/AR2006120401089.html |date=2008-10-28 }}", ''Washington Post'' 2006-12-05. Retrieved 2007-02-06.</ref>

== 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.<ref>Anika Rahman, Laura Katzive and Stanley K. Henshaw. "[http://www.guttmacher.org/pubs/journals/2405698.pdf A Global Review of Laws on Induced Abortion, 1985-1997] {{webarchive|url=https://web.archive.org/web/20160303230608/http://www.guttmacher.org/pubs/journals/2405698.pdf |date=2016-03-03 }}", ''International Family Planning Perspectives'' Volume 24, Number 2 (June 1998).</ref>

==Other animals==
{{further|Biology:Evolution of mammals}}
[[Image:Elefetusus.jpg|thumb|right|300px|Fourteen phases of elephant development before birth]]

A fetus is a stage in the prenatal development of viviparous organisms. This stage lies between [[Biology:Embryogenesis|embryogenesis]] and birth.<ref name=":0" /> Many vertebrates have fetal stages, ranging from most mammals to many fish. In addition, some invertebrates bear live young, including some species of [[Biology:Onychophora|onychophora]]<ref name=ony>{{Cite journal |year = 1995 |journal = Journal of Morphology |volume = 224 |pages = 179–198 |doi = 10.1002/jmor.1052240207 |pmid = 29865325 |title = Developing embryo and cyclic changes in the uterus of Peripatus (Macroperipatus) acacioi (Onychophora, Peripatidae) |author = Campiglia, Sylvia S. |last2 = Walker |first2 = Muriel H. |issue = 2 |s2cid = 46928727 }}</ref> and many [[Biology:Arthropod|arthropod]]s.

The fetuses of most mammals are situated similarly to the human fetus within their mothers.<ref>ZFIN, [http://zfin.org/zf_info/zfbook/stages/phar.html Pharyngula Period (24-48 h)] {{webarchive|url=https://web.archive.org/web/20070714230655/http://zfin.org/zf_info/zfbook/stages/phar.html |date=2007-07-14 }}. Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253-310. Downloaded 5 March 2007.</ref> However, the anatomy of the area surrounding a fetus is different in [[Biology:Litter (animal)|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, [[Earth:Predation|predators]]. [[Biology:Primate|Primate]]s are precocial at birth, with the exception of humans.<ref>Lewin, Roger. [https://books.google.com/books?id=SopsLRo1QyUC&pg=PA78 Human Evolution] {{Webarchive|url=https://web.archive.org/web/20230407212143/https://books.google.com/books?id=SopsLRo1QyUC&pg=PA78 |date=2023-04-07 }}, page 78 (Blackwell 2004).</ref>

The duration of gestation in [[Biology:Placentalia|placental mammals]] varies from 18 days in jumping mice to 23 months in [[Biology:Elephant|elephant]]s.<ref name="Sumich">Sumich, James and Dudley, Gordon. [https://books.google.com/books?id=TW2SAkgJOdMC&pg=PA320 Laboratory and Field Investigations in Marine Life], page 320 (Jones & Bartlett 2008).</ref> Generally speaking, fetuses of larger land mammals require longer gestation periods.<ref name="Sumich" />

[[Image:Walembryo 2.jpg|thumb|left|Fetal stage of a [[Biology:Porpoise|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.<ref name=ony/>
{{Clear}}

== See also ==
{|
|- style="vertical-align:top"
|style="padding-right:2em"|
* [[Biology:Fetal position|Fetal position]]
* [[Social:Fetal rights|Fetal rights]]
* [[Medicine:Fetoscopy|Fetoscopy]]
* Neural development
|
* [[Philosophy:Potential person|Potential person]]
* Pregnancy (mammals)
* Superfetation
* Women's rights
|}

== References ==
{{reflist|30em}}

== External links ==
{{Commons|Fetus}}
* [http://www.ehd.org/prenatal-images-index.php Prenatal Image Gallery Index] at the Endowment for Human Development website, featuring numerous motion pictures of human fetal movement.
* [https://web.archive.org/web/20080924185607/http://channel.nationalgeographic.com/episode/in-the-womb-2228?fs=www3.nationalgeographic.com#tab-Videos/01586_05 ''In the Womb''] (National Geographic video).
* [https://medlineplus.gov/ency/article/002398.htm Fetal development]: MedlinePlus Medical Encyclopedia

{{S-start}}
{{Succession box
 | title = Stages of human development
 | before = [[Biology:Embryo|Embryo]]
 | after = Infancy
 | years = Fetus
}}
{{S-end}}


[[Category:Animal developmental biology]]
[[Category:Embryology]]

{{Sourceattribution|Fetus}}