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| {{redirect|Unborn child|the Seals and Crofts album|Unborn Child (album)}} ] surrounds it.]] | |||
| A '''fetus''' (or '''foetus''', or '''fœtus''') is a developing ] or other ] ], after the ]nic stage and before ]. The plural is '''fetuses''' ('''foetuses''', '''fœtuses''') or, very rarely, ''foeti''. | |||
| In humans, the fetal stage of ] begins about eight weeks after ], when the major structures and ]s have formed, until birth.<ref>Some authorities suggest that the embryonic stage may last only seven weeks. See '''': "In humans, the organism is called an embryo for the first seven or eight weeks after conception, after which it is called a fetus." Also see '''', California Supreme Court: "beyond the embryonic stage of seven to eight weeks."</ref> | |||
| ==Etymology and spelling variations== | |||
| The word '''fetus''' is from the ] ''fetus'', meaning ''offspring'', ''bringing forth'', ''hatching of young''.<ref>Harper, Douglas. (2001). ''''. Retrieved ].</ref> It has ] roots related to sucking or ].<ref>'', Fourth Edition.'' Retrieved ].</ref> | |||
| '''Foetus''' is an English variation on the Latin spelling, and has been in use since at least 1594, according to the ], which describes "fetus" as the preferable spelling. Since the word "fetus" is not derived from the Latin verb ''foetare'', the superior etymological spelling does not include the letter '''o'''.<ref>Peters, P. (1998). The Cambridge Australian English Style Guide, Cambridge: Cambridge University Press. ISBN 0-521-57634-2</ref> The variant ''foetus'' or ''fœtus'' may have originated with an error by ], 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). | |||
| ==Human fetus== | |||
| ].]]The fetal stage begins eight weeks after fertilization. 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. Fetal growth can be terminated by various factors, including ], ] committed by a third party, or induced ]. | |||
| ===Development=== | |||
| The following timeline describes some of the specific changes in fetal ] and ] by fertilization age (i.e. the time elapsed since fertilization). However, it should be noted that obstetricians often use "]" which, by convention, is measured from 2 weeks earlier than fertilization. For purposes of this article, age is measured from fertilization, except as noted. | |||
| ; 8 weeks (condition at start of fetal stage) : The risk of ] decreases sharply at the beginning of the fetal stage.<ref> -. (August 6 , 2002). ''BBC News.'' Retrieved ].</ref> At this point, all major structures, including hands, feet, head, brain, and other organs are present, but they continue to grow, develop, and become more functional.<ref>'''' (Sixth Edition). Retrieved ].</ref> When the fetal stage commences, a fetus is typically about 30 mm (1.2 inches) in length, and the heart is beating.<ref>Greenfield, Marjorie. “". Retrieved ].</ref> The fetus bends the head, and also makes general ] and startles that involve the whole body.<ref name="Prechtl">Prechtl, Heinz. "" in ''Handbook of brain and behaviour in human development'', Kalverboer and Gramsbergen eds., pp. 415-418 (2001 Kluwer Academic Publishers). Retrieved ].</ref> ] activity has been detected as early as 54 days after conception.<ref name="Singer">Singer, Peter. '''', page 104 (St. Martins Press 1996). Retrieved ].</ref> Some fingerprint formation can be seen from the beginning of the fetal stage.<ref>Zabinski, Mark. Forensic Series Seminar, Pastore Chemical Laboratory, University of Rhode Island (February 2003) ( retrieved ]).</ref> (large image of fetus at 8 weeks after fertilization). Retrieved ]. A rotatable 3D version of this photo is available , and a sketch is available .</ref>]] | |||
| ; 8 to 15 weeks : The fetus continues to move in distinct motor patterns, picking up new patterns such as localized movement of the arms and legs, hiccups, breathing-like movements, and stretches and yawns .<ref name="Prechtl"/><ref>Butterworth, George and Harris, Margaret. '''' (Psychology Press 1994).</ref> The 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, '''' (2006). Retrieved ].</ref> At nine weeks, the fetus is able to bend fingers around an object; in response to a touch on the foot, the fetus will bend the legs or curl the toes to move away from the object.<ref>Valman, H. and Pearson, J. "", ''British Medical Journal'', (January 26, 1980). Retrieved ].</ref> The face is well-formed and develops a more human appearance. Eyelids close and remain closed for several months. The different appearance of the genitals in males and females becomes pronounced. ] buds appear, the ]s are long and thin, and ]s are produced in the ]. A fine hair called ] develops on the head. The gastrointestinal tract, still forming, starts to collect sloughed skin and lanugo, as well as hepatic products, forming ] (stool). Fetal ] is almost transparent. More muscle tissue and bones have developed, and the bones become harder. The first measurable signs of ] activity occur in the 12th week.<ref name="Singer"/><ref>Vogel, Friedrich. '''' (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 ].</ref> By the end of this stage, the fetus has reached about 15 cm (6 inches). (large image of fetus at 18 weeks after fertilization). Retrieved ]. A rotatable 3D version of this photo is available , and a sketch is available .</ref>]] | |||
| ; 16 to 25 weeks : The ] covers the entire body. Eyebrows, eyelashes, fingernails, and toenails appear. The fetus has increased muscle development. ] (air sacs) are forming in lungs. The ] develops enough to control some body functions. The ] are now developed, though the ] 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. The ], the first maternally discernable fetal movements, are often felt during this period. A woman pregnant for the first time (i.e. a primiparous woman) typically feels fetal movements at about 18-19 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. '''' (Blackwell 2000), page 8. Retrieved ].</ref> By the end of the fifth month, the fetus is about 20 cm (8 inches). (large image of fetus at 38 weeks aftre fertilization). Retrieved ]. A rotatable 3D version of this photo is available , and a sketch is available .</ref>]] | |||
| ; 26 to 38 weeks : The amount of body fat rapidly increases. Lungs are not fully mature. ] brain connections, which mediate sensory input, form. Bones are fully developed, but are still soft and pliable. ], ], and ] become more abundant. Continuous EEG readings have been observed by the 30th week.<ref name="Singer"/> Fingernails reach the end of the fingertips. The lanugo begins to disappear, until it is gone except on the upper arms and shoulders. Small ]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> ''BabyCenter.com'' Retrieved June 1, 2007.</ref> which means that the fetus is considered sufficiently developed for life outside the uterus.<ref>, retrieved ].</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'', '']l'', or ''fetal''.<ref name="Holden">Holden, Chris and MacDonald, Anita. '''' (Elsevier 2000). Retrieved ].</ref> | |||
| '''Maternal''' factors include maternal ], ], nutritional state, emotional ], toxin exposure (including ], ], ], and other drugs which can also harm the fetus in other ways), and ]. | |||
| '''Placental''' factors include size, microstructure (densities and architecture), ], transporters and binding proteins, nutrient utilization and nutrient production. | |||
| '''Fetal''' factors include the fetus genome, nutrient production, and ] 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. '''' (Blackwell 1999). Retrieved ].</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 (] shortly after birth), ], ], ], ], ], ] 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=== | |||
| Five months is currently the lower limit of ], and viability usually occurs later.<ref>Halamek, Louis. "", ''NeoReviews'', Vol.4 No.6 (2003): "most neonatologists would agree that survival of infants younger than approximately 22 to 23 weeks’ estimated gestational 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, expert postnatal care has improved with advances in medical science, and therefore the point of viability may have moved earlier.<ref>''Roe v. Wade'', (1973) ("viability is usually placed at about seven months (28 weeks) but may occur earlier, even at 24 weeks.") Retrieved ].</ref> As of 2006, the youngest child to survive a premature birth was ] at 21 weeks and 6 days' ].<ref>Baptist Hospital of Miami, (2006).</ref> | |||
| ===Fetal pain=== | |||
| {{main|Fetal pain}} | |||
| The subject of fetal ] and ] is controversial. The ability of a fetus to feel pain is often part of the ]. However, according to ], "there is no consensus among the medical and scientific experts about precisely when a fetus becomes pain-capable."<ref>Caplan, Arthur. Testimony to U.S. Congress, Subcommittee on the Constitution, Civil Rights, and Civil Liberties. "." (]). Retrieved ].</ref> Different sources have estimated that the earliest point for pain sensation may be during the first 12 weeks or after 20, 24, or 26 weeks gestation, or months after birth.{{Fact|date=October 2007}} | |||
| ===Circulatory system=== | |||
| ].]] | |||
| The ] of a human fetus works differently from that of born humans, mainly because the lungs are not in use: the fetus obtains ] and nutrients from the woman through the ] and the ].<ref name="Whitaker">Whitaker, Kent. '''' (Delmar 2001). Retrieved ].</ref> | |||
| Blood from the placenta is carried to the fetus by the ]. About half of this enters the fetal '']'' and is carried to the ], while the other half enters the ] 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 ]. The blood then moves to the right atrium of the ]. In the fetus, there is an opening between the right and left atrium (the '']''), and most of the blood flows from the right into the left atrium, thus bypassing ]. The majority of blood flow is into the left ventricle from where it is pumped through the ] 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 ] and other waste products from the fetus are taken up and enter the woman'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 ]. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the '']'', 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 ]).<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 "]"). More blood moves from the right atrium to the right ventricle and into the pulmonary arteries, and less flows through the '']'' 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 '']'' and the '']'' 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. '''' (Lippincott 2004). Retrieved ].</ref><ref>University of Michigan Medical School, . Retrieved ].</ref> | |||
| {| class="wikitable" | |||
| ! Fetal !! ] | |||
| |- | |||
| | ] || ] | |||
| |- | |||
| |] || ] | |||
| |- | |||
| | extra-] portion of the fetal left ] || ] (the "round ligament of the liver"). | |||
| |- | |||
| | intra-hepatic portion of the fetal left umbilical vein (the ]) || ] | |||
| |- | |||
| | ] portions of the fetal left and right ] || ] of the ] | |||
| |- | |||
| | ] portions of the fetal left and right umbilical arteries || ] (]) | |||
| |} | |||
| In addition to differences in circulation, the developing fetus also employs a different type of oxygen ] than adults (adults use ]). ] 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 ]s to permit sufficient time for the surgical correction of the anomalies. Conversely, in cases of ], 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 ] reduces the risk of ] and other ] 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 ], or other birth defects in the fetus. | |||
| During this time alcohol consumption may increase the risk of the development of ], a condition leading to ] 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|Fetal rights}} | |||
| Especially since the 1970s, there has been continuing debate over the "]" of the human fetus. Although ] of a fetus before ] is generally legal in the United States following the case of '']'', the third-party-killing of a fetus can be punishable as ] or ] throughout the pregnancy, depending on jurisdiction. | |||
| ==Non-human fetuses== | |||
| {{Expand-section|date=March 2007}}<!--Insufficient detail!--> | |||
| ]] | |||
| The fetus of most mammals develops similarly to the '']'' 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, . 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 ] 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}} | |||
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| ==See also== | |||
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| ==References== | |||
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Revision as of 16:45, 26 November 2007
Weyy x