Fetus
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226135219
2008-07-16T23:46:50Z
IronAngelAlice
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/* Weeks 11-17 */
{{otheruses|Fetus (disambiguation)}}
A '''fetus''' (or '''foetus''' or '''fœtus''') is a developing [[mammal]] or other [[viviparous]] [[vertebrate]], after the [[embryo]]nic stage and before [[childbirth|birth]]. The plural is '''fetuses''', or sometimes ''feti.'' The fetal stage of [[prenatal development]] starts when the major structures have formed, and lasts until birth.<ref>[http://www.medterms.com/script/main/art.asp?articlekey=3424 MedicineNet.com]: "The unborn offspring from the end of the 8th week after conception (when the major structures have formed) until birth." See also ''[http://www.bartleby.com/65/fe/fetus.html The Columbia Encyclopedia]'' (Sixth Edition). Retrieved [[2007-03-05]]: "the fetal stage begins seven to eight weeks after fertilization of the egg, when the embryo assumes the basic shape of the newborn and all the organs are present."</ref>
In humans, the fetal stage of [[prenatal development]] starts at 11th week in [[gestational age]] (the 9th week after [[fertilization]]).<ref name="nursing">Klossner, N. Jayne [http://books.google.com/books?id=B47OVg25g-QC&pg=PA103&lpg=PA103&dq=fetal+stage+begins&source=web&ots=dqQjWN-2jU&sig=-KVkuIJggNo1T_gV6AHkcc58xyI&hl=en Introductory Maternity Nursing] (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth"</ref> <ref>[http://www.americanpregnancy.org/duringpregnancy/fetaldevelopment1.htm The American Pregnancy Association]</ref>
==Etymology and spelling variations==
The word '''fetus''' is from the [[Latin]] ''fetus'', meaning ''offspring'', ''bringing forth'', ''hatching of young''.<ref>Harper, Douglas. (2001). ''[http://etymonline.com/ Online Etymology Dictionary]''. Retrieved [[2007-01-20]].</ref> It has [[Proto-Indo-European language|Indo-European]] roots related to sucking or [[suckling]].<ref>''[http://www.bartleby.com/61/roots/IE94.html The American Heritage Dictionary of the English Language], Fourth Edition.'' Retrieved [[2007-01-22]].</ref>
'''Fœtus''' is an English variation on the Latin spelling, and has been in use since at least 1594, according to the [[Oxford English Dictionary]], which describes "fœtus" as "incorrectly written". The variant ''fœtus'' may have originated with an error by [[Saint Isidore of Seville]], in AD 620.<ref name="BMJ"> {{cite journal | last = Aronson | first = Jeff | title = When I use a word...:Oe no! | journal = British Medical Journal | volume = 315 | issue = 1 | date = July 1997 | publisher = BMJ Publishing Group Ltd
| url = http://bmj.bmjjournals.com/cgi/content/full/315/7102/0/h | accessdate = 2006-06-29 }} </ref> The preferred spelling in the United States is ''fetus'', but the variants ''foetus'' and ''fœtus'' persist in other English-speaking countries and in some medical contexts, as well as in some other languages (e.g., French). In technical usage, ''fetus'' is now the standard spelling throughout the English-speaking world.
==Human fetus==
[[Image:Views of a Foetus in the Womb detail.jpg|thumb|left|Selection from "Views of a Fetus in the Womb", a drawing by [[Leonardo da Vinci]].]]The fetal stage starts at the beginning of the 9th week following [[fertilization]], after the [[zygote]], [[blastocyst]], and [[embryo]]nic stages. The risk of miscarriage decreases sharply at the beginning of the fetal stage.<ref>• [http://news.bbc.co.uk/2/hi/health/2176898.stm Q&A: Miscarriage]. (August 6 , 2002). ''BBC News.'' Retrieved [[2007-04-22]]: “The risk of miscarriage lessens as the pregnancy progresses. It decreases dramatically after the 8th week.”<br>
• [[Lennart Nilsson]], A Child is Born 91 (1990): at eight weeks, "the danger of a miscarriage … diminishes sharply."<br>
• “[http://www.womens-health.co.uk/miscarr.asp Women’s Health Information]”, Hearthstone Communications Limited: “The risk of miscarriage decreases dramatically after the 8th week as the weeks go by.” Retrieved [[2007-04-22]].</ref> The fetus is not as sensitive to damage from environmental exposures as the embryo was, though toxic exposures can often cause physiological abnormalities or minor congenital malformation.{{Fact|date=March 2008}} Fetal growth can be terminated by various factors, including [[miscarriage]], [[feticide]] committed by a third party, or induced [[abortion]].
===Development===
The following describes some of the specific changes in fetal [[anatomy]] and [[physiology]] by fertilization age (i.e. the time elapsed since fertilization). Obstetricians often use "[[gestational age]]" which, by convention, is measured from 2 weeks earlier than fertilization. For purposes of this article, age is measured from gestation rather than from fertilization, except as noted.
====Weeks 11-17====
[[Image:10 weeks pregnant.png|thumb|right|Artist's depiction of fetus in the 11th week. The [[crown-rump length]] is 1.25 inches.<ref>Marc H. Bornstein, Michael E. Lamb. [http://books.google.com/books?num=100&hl=en&safe=off&q=%22crown+to+rump%22+fetus&um=1&ie=UTF-8&sa=N&tab=wp Developmental Science: An Advanced Textbook], page 227 (2005): "At 8 weeks, fetuses measure 3.18 cm from crown to rump (1.25 inches)."</ref>]] The fetal stage commences at the beginning of the 11th week.<ref name="nursing" /> 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.<ref name="nursing" /> The head makes up nearly half of the fetus' size.<ref name = "itrfps">[http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm MedlinePlus]</ref> Breathing-like movement of the fetus is necessary for stimulation of 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]'' (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]'' (Sixth Edition). Retrieved [[2007-03-05]].</ref><ref>Greenfield, Marjorie. “[http://www.drspock.com/article/0,1510,9851,00.html Dr. Spock.com]". Retrieved [[2007-01-20]].</ref>
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.<ref name="JAMA"> {{cite journal | last = Lee | first = Susan | title = Fetal Pain A Systematic Multidisciplinary Review of the Evidence | journal = The Journal of the American Medical Association | volume = 294 | issue = 8 | date = August 24/31, 2005 | publisher = the American Medical Association | url = http://jama.ama-assn.org/cgi/content/full/294/8/947 | accessdate = 2008-02-14 | pmid = 16118385 | doi = 10.1001/jama.294.8.947 | pages = 947 }} (''see [[Fetus#Fetal_pain|Fetal Pain]] section'') </ref><ref name = "sskqke">[http://www.msnbc.msn.com/id/9053416/ "Study: Fetus feels no pain until third trimester"] MSNBC</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" />
From weeks 11 to 14, the fetal eyelids close and remain closed for several months, and the fetus' sex may be apparent.<ref name="medline">[http://www.mayoclinic.com/health/prenatal-care/PR00112 ''Mayo Clinic'']</ref> [[Tooth]] buds appear, the [[Limb (anatomy)|limb]]s are long and thin, and [[red blood cell]]s are produced in the [[liver]], however the majority of red blood cells will be made later in gestation (at 21 weeks) by bone marrow.<ref name = "itrfps"/> A fine hair called [[lanugo]] develops on the head. The gastrointestinal tract, still forming, starts to collect sloughed skin and lanugo, as well as hepatic products, forming [[meconium]] (stool).<ref name = "itrfps"/> Fetal [[skin]] is almost transparent. The first measurable signs of [[Electroencephalography|EEG]] movement occur in the 12th week.<ref>Vogel, Friedrich. ''[http://books.google.com/books?vid=ISBN3540655735&id=r0lXOLhd-kAC&pg=PA13&lpg=PA13&dq=%22EEG+activity%22+and+fetus+and+brain&sig=6uMfRi6Ql5sW3jBLo6L05h0i1UI Genetics and the Electroencephalogram]'' (Springer 2000): "Slow EEG activity (0.5 – 2 c/s) can be demonstrated in the fetus even at the conceptual age of three months." Retrieved [[2007-03-05]].</ref>
====Weeks 18-27====
[[Image:20 weeks pregnant.png|thumb|left|Artist's depiction of fetus at 20 weeks, about 6.5 inches crown to rump.]]The [[lanugo]] covers the entire body. Eyebrows, eyelashes, fingernails, and toenails appear. The fetus has increased muscle development. [[Alveoli]] (air sacs) are forming in lungs. The [[nervous system]] develops enough to control some body functions. The [[cochlea]] are now developed, though the [[myelin]] sheaths in the neural portion of the auditory system will continue to develop until 18 months after birth. The respiratory system has developed to the point where gas exchange is possible. A woman pregnant for the first time (i.e. a primiparous woman) typically feels fetal movements at about 20-21 weeks, whereas a woman who has already given birth at least two times (i.e. a multiparous woman) will typically feel movements around 16 weeks.<ref>Levene, Malcolm et al. ''[http://books.google.com/books?vid=ISBN0632051639&id=FHXtDkLzOHEC&pg=RA2-PA8&lpg=RA2-PA8&ots=fkQG1qvn5i&dq=%22quickening%22+and+obese&num=100&sig=-Xgmp3WR71poUh1qJ3oke-5XTOM#PRA2-PA8,M1 Essentials of Neonatal Medicine]'' (Blackwell 2000), page 8. Retrieved [[2007-03-04]].</ref> By the end of the fifth month, the fetus is about 20 cm (8 inches).
====Weeks 28-40====
[[Image:40 weeks pregnant.png|thumb|right|Artist's depiction of fetus at 40 weeks, about 20 inches head to toe.]]The amount of body fat rapidly increases. Lungs are not fully mature. [[thalamus|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 bud]]s are present on both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 38th week. The fetus is considered full-term between weeks 35 and 40,<ref>[http://www.babycenter.com/mybabycenter/136.html Your Pregnancy: 36 Weeks] ''BabyCenter.com'' Retrieved [[June 1]] [[2007]].</ref> which means that the fetus is considered sufficiently developed for life outside the uterus.<ref>[http://www.wordwebonline.com/en/FULLTERM Word Web Online], retrieved [[2007-01-26]].</ref> It may be 48 to 53 cm (19 to 21 inches) in length, when born.
====Variation in growth====
{{seealso|Birth weight}}
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'', ''[[placenta]]l'', or ''fetal''.<ref name="Holden">Holden, Chris and MacDonald, Anita. ''[http://books.google.com/books?vid=ISBN070202421X&id=EUXbcmfcADYC&pg=RA2-PA174&lpg=RA2-PA174&ots=_-wbxFW87C&dq=%22growth%22+and+variation+and+fetus+and+%22placental+factors%22+and+%22maternal+factors%22+and+%22fetal+factors%22&num=100&sig=0jIpH8JKU_2TDllfpRuCjcFgD1I Nutrition and Child Health]'' (Elsevier 2000). Retrieved [[2007-03-04]].</ref>
'''Maternal''' factors include maternal [[weight]], [[body mass index]], nutritional state, emotional [[Stress (medicine)|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.<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. ''[http://books.google.com/books?vid=ISBN0632043326&id=sfp-OzoU6X4C&pg=RA2-PA6&lpg=RA2-PA6&ots=otwaM0KidM&dq=sga+aga+lga+growth+factors+placental&num=100&sig=bzteZcizgLWVNg6heQ7QWnLur3A#PRA2-PA6,M1 Management of High-Risk Pregnancy]'' (Blackwell 1999). Retrieved [[2007-03-04]].</ref> 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===
The lower limit of [[viability]] is approximately five months [[gestational age]], and usually later.<ref>Halamek, Louis. "[http://neoreviews.aappublications.org/cgi/content/extract/4/6/e153 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."</ref> According to ''The Developing Human'':
<blockquote>Viability is defined as the ability of fetuses to survive in the extrauterine environment... There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable or beyond which survival is assured, but experience has shown that it is rare for a baby to survive whose weight is less than 500 gm or whose fertilization age is less than 22 weeks. Even fetuses born between 26 and 28 weeks have difficulty surviving, mainly because the respiratory system and the central nervous system are not completely differentiated... If given expert postnatal care, some fetuses weighing less than 500 gm may survive; they are referred to as ''extremely low birth weight'' or ''immature infants''.... Prematurity is one of the most common causes of morbidity and prenatal death.<ref>{{cite book |author=Moore, Keith and Persaud, T. |title=The Developing Human: Clinically Oriented Embryology' |publisher=Saunders |location=Philadelphia |year=2003 |isbn=0-7216-9412-8 |pages=p. 103}}</ref></blockquote>
During the past several decades, neonatal care has improved with advances in medical science, and therefore the point of viability may have moved earlier.<ref>''Roe v. Wade'', [http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=CASE&court=US&vol=410&page=113 410 U.S. 113] (1973) ("viability is usually placed at about seven months (28 weeks) but may occur earlier, even at 24 weeks.") Retrieved [[2007-03-04]].</ref> As of 2006, the [[Premature birth#Records|two youngest children]] to survive premature birth are thought to be James Elgin Gill (born on [[20 May]] [[1987]] in [[Ottawa]], [[Canada]], at 21 weeks and 5 days gestational age),<ref name="titlePowell's Books - Guinness World Records 2004 (Guinness Book of Records) by">{{cite web |url=http://www.powells.com/biblio?show=0553587129&page=excerpt? |title=Powell's Books - Guinness World Records 2004 (Guinness Book of Records) by |accessdate=2007-11-28 |format= |work=}}</ref><ref name="titleMiracle child">{{cite web |url=http://www.canada.com/topics/bodyandhealth/story.html?id=db8f33ab-33e9-429f-bedc-b6ca80f61bdc |title=Miracle child |accessdate=2007-11-28 |format= |work=}}</ref> and Amillia Taylor (born on [[24 October]] [[2006]] in [[Miami]], [[Florida]], at 21 weeks and 6 days gestational age).<ref name="titletrithuc.thanhnienkhcn.org.vn">{{cite web |url=http://210.245.61.173/en/detail.php? |title=trithuc.thanhnienkhcn.org.vn |accessdate=2007-11-28 |format= |work=}}</ref><ref name="bbcAmillia">{{cite news | url = http://news.bbc.co.uk/1/hi/world/americas/6384621.stm | title = Most-premature baby allowed home | publisher = BBC News | date = [[2007-02-21]] | accessdate=2007-05-05 }}</ref><ref>Baptist Hospital of Miami, [http://www.baptisthealth.net/vgn/images/portal/cit_449/59/45/73662064factsheetTaylorbaby.pdf Fact Sheet] (2006).</ref> Both children were born just under 20 weeks from fertilization, or a few days past the midpoint of an average full-term pregnancy. Despite their premature births, both developed into healthy children.
===Fetal pain===
{{main|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."<ref name="JAMA"> {{cite journal | last = Lee | first = Susan | title = Fetal Pain A Systematic Multidisciplinary Review of the Evidence | journal = The Journal of the American Medical Association | volume = 294 | issue = 8 | date = August 24/31, 2005 | publisher = the American Medical Association
| url = http://jama.ama-assn.org/cgi/content/full/294/8/947 | accessdate = 2008-02-14 | pmid = 16118385 | doi = 10.1001/jama.294.8.947 | pages = 947 }} 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, [http://www.nytimes.com/2005/08/26/health/26pain.html “Study Authors Didn't Report Abortion Ties”], ''New York Times'' ([[2005-08-26]]).</ref><ref name = "sskqke"/> However, there may be an emerging consensus among developmental [[neurobiology|neurobiologists]] that the establishment of [[Human thalamus|thalamocortical]] connections" (at about 26 weeks) is a critical event with regard to fetal perception of pain.<ref name="Johnson">Johnson, Martin and Everitt, Barry. ''[http://books.google.com/books?vid=ISBN0632042877&id=MzZRuSQ5UeEC&pg=PA215&lpg=PA215&ots=cx0KcmuOYk&dq=%22emerging+consensus+among+developmental+neurobiologists+that+the+establishment+%22&num=100&sig=8I9DY9KPpuSPNYvGI3sEV2bmKsA 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> 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.<ref name="Johnson"/>
Whether a fetus has the ability to feel [[pain]] and to [[suffering|suffer]] is part of the [[abortion debate]].<ref>White, R. Frank. "[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]", ''Illinois Bar Journal'' (December 2002). Retrieved [[2007-03-10]].</ref> For example, in the USA legislation has been proposed by [[pro-life]] advocates requiring abortion providers to tell a woman that the fetus may feel pain during the abortion procedure, and that require her to accept or decline anesthesia for the fetus.<ref>Weisman, Jonathan. "[http://www.washingtonpost.com/wp-dyn/content/article/2006/12/04/AR2006120401089.html House to Consider Abortion Anesthesia Bill]", ''Washington Post'' [[2006-12-05]]. Retrieved [[2007-02-06]].</ref>
===Fetal movement===
{{main|Fetal movement}}
The parts of the fetal brain that control movement will not fully form until late in the second trimester, and the first part of the third trimester.<ref>[http://linkinghub.elsevier.com/retrieve/pii/S1744165X06000680 The development of cerebral connections during the first 20–45 weeks’ gestation]. Seminars in Fetal and Neonatal Medicine, Volume 11, Issue 6, Pages 415-422</ref> Control of movement is limited at birth, and purposeful voluntary movements develop in the first year after birth.<ref>Stanley, Fiona et al. [http://books.google.com/books?id=aJwvMYMAfmYC&pg=PA48&lpg=PA48&dq=%22Voluntary+control+of+movement+develops+and+matures+%22&source=web&ots=JhdiM8qqIj&sig=U1BxRZKZANvTlSY35V7pc6dVmJg&hl=en "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...."</ref><ref name="Becher">Becher, Julie-Claire. [http://www.behindthemedicalheadlines.com/articles/insights-into-early-fetal-development "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): "Purposive movement depends on brain maturation. This begins at about 18 weeks and progressively replaces reflex movements, which disappear by about 8 months after birth....Reflexes are very different from purposeful voluntary movements which develop during the first year of life."</ref>
Locomotor activity begins during the late [[embryo|embryonic stage]], and changes in nature throughout [[prenatal development|development]]. Muscles begin to move as soon as they are [[nerve|innervated]]. These first movements are not reflexive, but arise from nerve impulses originating in the [[spinal cord]]. As the [[nervous system]] matures, muscles can move in response to stimuli, though this is not a voluntary movement.<ref>Vaughan 1996, p. 208.</ref>
[[Quickening]] is the first maternally discernible fetal movement, which is often felt around the middle of pregnancy. Women who have already given birth have more relaxed uterine muscles that are consequently more sensitive to fetal motion, and for them fetal motion can sometimes be felt as early as 14 weeks.<ref>Van Der Ziel, Cornelia & Tourville, Jacqueline. ''[http://books.google.com/books?vid=ISBN1569243190&id=jciZAfJWoqsC&pg=RA1-PA70&lpg=RA1-PA70&ots=RD_hiLptXz&dq=quickening+and+%22plus-size%22&sig=S9lPp8zEcLGW3cyEnCouefDFZxI Big, Beautiful & Pregnant: Expert Advice And Comforting Wisdom for the Expecting Plus-size Woman]'' (Marlowe 2006). Retrieved [[2007-02-15]].</ref>
Complex and generalized movements occur at the beginning of the fetal stage, with movements and startles that involve the whole body. These movements occur as muscles and neural pathways begin to develop<ref name="Prechtl">Prechtl, Heinz[http://books.google.com/books?vid=ISBN0792369432&id=FzyPozUyKPkC&pg=RA1-PA416&lpg=RA1-PA416&dq=fetus+and+movement&num=100&sig=6_E9lwpo1KhTtwzIkTKh2difcbo#PRA1-PA415,M1 "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."</ref> Movement of hands, hips and knees have been observed at nine weeks,<ref>Valman, H. and Pearson, J. "[http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1600041 What the Fetus Feels]", ''British Medical Journal'', ([[January 26]] [[1980]]). Retrieved [[2007-03-04]]: "Nine weeks after conception...fingers [bend] round an object in the palm of his hand. In response to a touch on the sole of his foot...hips and knees [bend] to move away from the touching object."</ref> stretches and yawns at ten weeks,<ref>Butterworth, George and Harris, Margaret. ''[http://books.google.com/books?vid=ISBN0863772803&id=P6Bp3Ysuc58C&pg=PA48&lpg=PA48&ots=qCzYqGfIka&dq=%22stretch+and+yawn%22+and+fetus&sig=376BGiAWv-zXRPp9XE15GeCDB6I Principles of developmental psychology]'', page 48 (Psychology Press 1994): "stretch and yawn pattern at 10 weeks."</ref> and isolated limb movements beginning shortly thereafter.<ref name="Prechtl">Prechtl, Heinz. [http://books.google.com/books?vid=ISBN0792369432&id=FzyPozUyKPkC&pg=RA1-PA416&lpg=RA1-PA416&dq=fetus+and+movement&num=100&sig=6_E9lwpo1KhTtwzIkTKh2difcbo#PRA1-PA415,M1 "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): "At 9-10 weeks postmenstrual age complex and generalized movements occur... [I]solated movements of one arm or leg emerge 1 week later."</ref>
===Circulatory system===
[[Image:Fetal circulation.png|thumb|right|220px|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 mother through the [[placenta]] and the [[umbilical cord]].<ref name="Whitaker">Whitaker, Kent. ''[http://books.google.com/books?vid=ISBN0766813738&id=R3WK8XyAHYgC&pg=PA18&lpg=PA18&ots=wcft0RkRxg&dq=%22fetal+circulatory+system%22&sig=_DSt-D7pBSBtikfJ5oQKYVlNKmU Comprehensive Perinatal and Pediatric Respiratory Care]'' (Delmar 2001). 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 [[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 (heart)|foramen ovale]]''), and most of the blood flows through this hole directly into the left atrium from the right atrium, thus bypassing [[pulmonary circulation]]. The continuation of this blood flow is into the left ventricle, and from there 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.<ref name="Whitaker"/>
Some of the blood entering the right atrium does not pass directly to the left atrium through the ''[[foramen ovale (heart)|foramen ovale]]'', 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]]).<ref name="Whitaker"/>
====Postnatal development====
{{Main|Adaptation to extrauterine life}}
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 (heart)|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:<ref>Dudek, Ronald and Fix, James. ''[http://books.google.com/books?vid=ISBN0781757266&id=LGcI-FVCruEC&pg=RA1-PA54&lpg=RA1-PA54&ots=2h9BlGiwkB&dq=%22Remnants+of+fetal+circulatory%22&sig=FFhRCd92q-wfUbGgR81vu62sBMA#PRA1-PA54,M1 Board Review Series Embryology]'' (Lippincott 2004). Retrieved [[2007-03-04]].</ref><ref>University of Michigan Medical School, [http://www.med.umich.edu/lrc/coursepages/M1/embryology/embryo/18changesatbirth.htm Fetal Circulation and Changes at Birth]. Retrieved [[2007-03-04]].</ref>
{| class="wikitable" style="background:white;"
! Fetal !! [[Adult]]
|-
| [[foramen ovale]] || [[Fossa ovalis (heart)|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 protein|transport molecule]] than adults (adults use [[hemoglobin|adult hemoglobin]]). [[Fetal hemoglobin]] enhances the fetus' ability to draw oxygen from the placenta. Its association 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===
{{seealso|Congenital disorder}}
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 [[prostaglandin]]s 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 <ref> Dalby, JT. (1978).''Environmental effects on prenatal development'' Journal of Pediatric Psychology, 3, 105-109. </ref> 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.<ref>Streissguth, A. (1997). ''Fetal Alcohol Syndrome: A Guide for Families and Communities''. Baltimore: Brookes Publishing. ISBN 1-55766-283-5.</ref>
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===
{{main|Abortion Debate|Fetal rights}}
In the United States, some states have laws that impose strict punishments for those who inflict violence that results in damage to a fetus or the unwanted termination of a pregnancy. The severity of the punishment, and the stage of fetal development where laws start to apply vary from state to state.<ref>National Conference of State Legislatures. (June 2006). "[http://www.ncsl.org/programs/health/fethom.htm Fetal Homicide]". Retrieved [[January 19]] [[2007]].</ref>
[[Abortion law|Abortion of a fetus is legal]] in many countries such as Australia, Canada, Mexico, UK and USA. Many of those countries that allow abortion during the fetal stage have gestational time limits, so that [[Late-term abortion#Legal restrictions on later abortion|late-term abortions are not normally allowed]].<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], International Family Planning Perspectives (Volume 24, Number 2, June 1998).</ref>
==Non-human fetuses==
{{Expand-section|date=March 2007}}<!--Insufficient detail!-->
[[Image:e17-fetus.gif|thumb|right|[[Histology|Histological]] slice of a rat fetus at E17 from [http://brainmaps.org/index.php?action=viewslides&datid=89 BrainMaps.org] ]]
The fetus of most mammals develops similarly to the ''[[Homo sapiens]]'' fetus. After the first stages of development, the human embryo reaches a stage very similar to all other vertebrates<!--Technically chordates, but this is a little too much detail-->.<ref>ZFIN, [http://zfin.org/zf_info/zfbook/stages/phar.html Pharyngula Period (24-48 h)]. Modified from: Kimmel et al., 1995. Developmental Dynamics 203:253-310. Downloaded 5 March 2007.</ref> The anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus is surrounded by placental [[Biological tissue|tissue]] and is lodged along one of two long uteri instead of the single uterus found in a human female. Development at birth is similar, with animals also having a poorly developed sense of vision and other senses.{{Fact|date=March 2007}}
{{-}}
==See also==
{{commons|Fetus}}
* [[Prenate]]
* [[Embryo]]
* [[Pregnancy]]
* [[Child]]
* [[Superfetation]]
* [[Neural development]]
* [[Fetoscopy]]
* [[Fetal position]]
* [[Fetal rights]]
* [[Women's rights]]
==References==
{{reflist|colwidth=35em}}
==External links==
* [http://brainmaps.org/index.php?action=viewslides&datid=89 High Resolution Nissl-stained Rat Fetus]
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{{succession box|title=[[Stages of human development]] |before=[[Embryo]]|after=[[Infanthood|Infant]]|years=Fetus}}
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{{Embryology}}
[[Category:Developmental biology]]
[[Category:Embryology]]
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