A fetus (or foetus or fœtus) is a developing
mammalor other viviparous vertebrate, after the embryonic stage and before birth. The plural is fetuses, or sometimes "feti." The fetal stage of prenatal developmentstarts when the major structures have formed, and lasts until birth. [ [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."]
In humans, the fetal stage of
prenatal developmentstarts at the beginning of the 11th week in gestational age(the 9th week after fertilization).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"] [ [http://www.americanpregnancy.org/duringpregnancy/fetaldevelopment1.htm The American Pregnancy Association] ]
Etymology and spelling variations
The word fetus is from the
Latin"fetus", meaning "offspring", "bringing forth", "hatching of young". [Harper, Douglas. (2001). " [http://etymonline.com/ Online Etymology Dictionary] ". Retrieved 2007-01-20.] It has Indo-European roots related to sucking or suckling. [" [http://www.bartleby.com/61/roots/IE94.html The American Heritage Dictionary of the English Language] , Fourth Edition." Retrieved 2007-01-22.]
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. 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 ] 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.
: “The risk of miscarriage lessens as the pregnancy progresses. It decreases dramatically after the 8th week.”
Lennart Nilsson, A Child is Born 91 (1990): at eight weeks, "the danger of a miscarriage … diminishes sharply."
• “ [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.] 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, feticidecommitted by a third party, or induced abortion.
The following describes some of the specific changes in fetal
anatomyand physiologyby 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.
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. 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 Fetal Pain section") ] [http://www.msnbc.msn.com/id/9053416/ "Study: Fetus feels no pain until third trimester"] MSNBC] At this point in development, uncontrolled movements and twitches occur as muscles, the brain and pathways begin to develop.
From weeks 11 to 14, the fetal eyelids close and remain closed for several months, and the fetus' sex may be apparent. [http://www.mayoclinic.com/health/prenatal-care/PR00112 "Mayo Clinic"] ]
Toothbuds appear, the limbs are long and thin, and red blood cells are produced in the liver, however the majority of red blood cells will be made later in gestation (at 21 weeks) by bone marrow. A fine hair called lanugodevelops on the head. The gastrointestinal tract, still forming, starts to collect sloughed skin and lanugo, as well as hepatic products, forming meconium(stool). Fetal skinis almost transparent. The first measurable signs of EEG movement occur in the 12th week. [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.]
.] By the end of the fifth month, the fetus is about 20 cm (8 inches).
.] It may be 48 to 53 cm (19 to 21 inches) in length, when born.
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".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.]
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 growth is often classified as follows: small for gestational age (SGA), appropriate for gestational age (AGA), and large for gestational age (LGA). [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.] 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 ( deathshortly after birth), asphyxia, hypothermia, polycythemia, hypocalcemia, immune dysfunction, neurologicabnormalities, and other long-term health problems. SGA may be associated with growth delay, or it may instead be associated with absolute stunting of growth.
The lower limit of
viabilityis approximately five months gestational age, and usually later. [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."] According to "The Developing Human":
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. [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]
During the past several decades, neonatal care has improved with advances in medical science, and therefore the point of viability may have moved earlier. ["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.] As of 2006, the two youngest children to survive premature birth are thought to be James Elgin Gill (born on 20 May 1987in Ottawa, Canada, at 21 weeks and 5 days gestational age),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=] 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=] and Amillia Taylor (born on 24 October 2006in Miami, Florida, at 21 weeks and 6 days gestational age).cite web |url=http://220.127.116.11/en/detail.php? |title=trithuc.thanhnienkhcn.org.vn |accessdate=2007-11-28 |format= |work=] 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 ] [Baptist Hospital of Miami, [http://www.baptisthealth.net/vgn/images/portal/cit_449/59/45/73662064factsheetTaylorbaby.pdf Fact Sheet] (2006).] 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, 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." 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).] 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.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.] 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.
Whether a fetus has the ability to feel
painand to suffer is part of the abortion debate. [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.] [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.] For example, in the USA legislation has been proposed by pro-lifeadvocates 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. [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.]
Quickeningis 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 may be felt as early as 18 weeks. [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.] Mothers can begin to feel quickening anywhere between 18 and 24 weeks of gestation. [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.]
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. [ [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] Control of movement is limited at birth, and purposeful voluntary movements develop in the first year after birth. [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...."] 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)] However, locomotor activity begins during the late embryonic stage, and changes in nature throughout development. Muscles begin to move as soon as they are innervated. These first movements are not reflexive, but arise from nerve impulses originating in the
spinal cord. As the nervous systemmatures, muscles can move in response to stimuli, though this is not a voluntary movement. [Vaughan 1996, p. 208.] 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)] [Valman, H. and Pearson, J. " [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1600041 What the Fetus Feels] ", "British Medical Journal", ( January 26 1980). ] [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."] 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)]
circulatory systemof a human fetus works differently from that of born humans, mainly because the lungs are not in use: the fetus obtains oxygenand nutrients from the mother through the placentaand the umbilical cord.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.]
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.
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 liverproper 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 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 aortainto 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 dioxideand other waste products from the fetus are taken up and enter the woman's circulation.
Some of the blood entering the right atrium does not pass directly to the left atrium through the "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).
Differences from the adult circulatory system
Remnants of the fetal circulation can be found in adults: [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.] [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.]
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 hemoglobinenhances 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.
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 [ Dalby, JT. (1978)."Environmental effects on prenatal development" Journal of Pediatric Psychology, 3, 105-109. ] Diet is especially important in the early stages of development. Studies show that supplementation of the woman's diet with
folic acidreduces the risk of spina bifidaand other neural tubedefects. 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 retardationin some infants. [Streissguth, A. (1997). "Fetal Alcohol Syndrome: A Guide for Families and Communities". Baltimore: Brookes Publishing. ISBN 1-55766-283-5.] 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.
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. [National Conference of State Legislatures. (June 2006). " [http://www.ncsl.org/programs/health/fethom.htm Fetal Homicide] ". Retrieved
January 19 2007.]
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 abortions are not normally allowed. [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).]
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 . [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.] The anatomy of the area surrounding a fetus is different in litter-bearing animals compared to humans: each fetus 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 is similar, with animals also having a poorly developed sense of vision and other senses.Fact|date=March 2007
* [http://brainmaps.org/index.php?action=viewslides&datid=89 High Resolution Nissl-stained Rat Fetus]
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