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For other uses, see Foetus (band).
"Unborn child" redirects here. For the Seals and Crofts album, see Unborn Child (album).
Human fetus at eight weeks. A small part of the placenta is shown at the bottom, while the fluid-filled amnion surrounds it.
A fetus (or foetus or fœtus) is a developing mammal or other viviparous vertebrate, after the embryonic stage and before birth. The plural is fetuses, or sometimes feti.
In humans, the fetal stage of prenatal development starts at approximately the end of the eighth week after fertilization, when the major structures have formed,MedicineNet.com: "The unborn offspring from the end of the 8th week after conception (when the major structures have formed) until birth." See also Dictionary.com: "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." and lasts until birth.Some authorities suggest that the embryonic stage may last only seven weeks. See Encyclopedia Britannica: "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 People v. Taylor, California Supreme Court: "beyond the embryonic stage of seven to eight weeks."
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The word fetus is from the Latin fetus, meaning offspring, bringing forth, hatching of young.Harper, Douglas. (2001). Online Etymology Dictionary. Retrieved 2007-01-20. It has Indo-European roots related to sucking or suckling.The American Heritage Dictionary of the English Language, Fourth Edition. Retrieved 2007-01-22.
Foetus 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 "fetus" as "etymologically preferable ... but in actual use ... almost unknown", and gives foetus as the standard spelling. The variant foetus or fœtus may have originated with an error by Saint Isidore of Seville, in AD 620. Aronson, Jeff (July 1997). "When I use a word...:Oe no!". British Medical Journal 315 (1). BMJ Publishing Group Ltd. Retrieved on 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).
The fetal stage starts at the beginning of the 9th week following fertilization, after the blastocyst, zygote, and embryonic stages. The risk of miscarriage decreases sharply at the beginning of the fetal stage.• 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.”
• Lennart Nilsson, A Child is Born 91 (1990): at eight weeks, "the danger of a miscarriage … diminishes sharply."
• “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.[citation needed] Fetal growth can be terminated by various factors, including miscarriage, feticide committed by a third party, or induced abortion.
3D ultrasound of 3-inch fetus (about 12 weeks after fertilization) |
3D ultrasound at 17 weeks |
3D ultrasound at 20 weeks |
The following chronology describes some of the specific changes in fetal anatomy and physiology by fertilization age (i.e. the time elapsed since fertilization). However, certain specific aspects of development such as viability, fetal pain, and fetal movement are discussed in subsequent sections after this chronology.
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 fertilization rather than from two weeks earlier, except as noted.
Artist\'s depiction of fetus 8 weeks after fertilization. The crown-rump length is 1.25 inches.Marc H. Bornstein, Michael E. Lamb. Developmental Science: An Advanced Textbook, page 227 (2005): "At 8 weeks, fetuses measure 3.18 cm from crown to rump (1.25 inches)."
The fetal stage commences at the beginning of the 9th week after fertilization (i.e. at the beginning of the 11th week of gestational age).Introductory to Maternal Nursing "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth" At the start of the fetal stage, the fetus is typically about 30 mm (1.2 inches) in length from crown to rump, and the head makes up nearly half of the fetus\' size.MedlinePlus Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.Institute of Medicine of the National Academies, Preterm Birth: Causes, Consequences, and Prevention (2006), page 317. Retrieved 2008-03-12: "Fetal breathing movements begin as early as 10 weeks of gestation, and the breathing of amniotic fluid in and out is essential for the stimulation of lung development." The heart is beating.Greenfield, Marjorie. “Dr. Spock.com". Retrieved 2007-01-20.The heart, hands, feet, brain and other organs are present, but are not developed sufficiently for the fetus to survive on its own.The Columbia Encyclopedia (Sixth Edition). Retrieved 2007-03-05. The fetus is surrounded by amniotic fluid which offers protection and allows room for movement. The pregnant woman\'s placenta and umbilical cord provide oxygen, nutrients, and allow waste elimination.
From weeks 9 to 12, the fetal eyelids close and remain closed for several months, and the appearance of the genitals in males and females becomes more apparent.Medline Plus Medical Encyclopedia Tooth buds 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.MedlinePlus 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).MedlinePlus Fetal skin is almost transparent. The first measurable signs of EEG movement occur in the 12th week.Vogel, Friedrich. 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 this stage, the fetus has reached about 15 cm (6 inches).
Artist\'s depiction of fetus at 18 weeks after fertilization, 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 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.Levene, Malcolm et al. Essentials of Neonatal Medicine (Blackwell 2000), page 8. Retrieved 2007-03-04. By the end of the fifth month, the fetus is about 20 cm (8 inches).
Artist\'s depiction of fetus at 38 weeks after fertilization, about 20 inches head to toe.
The amount of body fat rapidly increases. Lungs are not fully mature. Thalamic brain connections, which mediate sensory input, form. Bones are fully developed, but are still soft and pliable. Iron, calcium, and phosphorus become more abundant. Fingernails reach the end of the fingertips. The lanugo begins to disappear, until it is gone except on the upper arms and shoulders. Small breast buds are present on both sexes. Head hair becomes coarse and thicker. Birth is imminent and occurs around the 38th week. The fetus is considered full-term between weeks 35 and 40,Your Pregnancy: 36 Weeks BabyCenter.com Retrieved June 1, 2007. which means that the fetus is considered sufficiently developed for life outside the uterus.Word Web Online, retrieved 2007-01-26. It may be 48 to 53 cm (19 to 21 inches) in length, when born.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. 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 factors include the fetus genome, nutrient production, and hormone output. Also, female fetuses tend to weigh less than males, at full term.
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. 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 (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.
The lower limit of viability is approximately five months gestational age, and usually later.Halamek, Louis. "Prenatal Consultation at the Limits of Viability", NeoReviews, Vol.4 No.6 (2003): "most neonatologists would agree that survival of infants younger than approximately 22 to 23 weeks’ estimated gestational age [i.e. 20 to 21 weeks\' estimated fertilization age] is universally dismal and that resuscitative efforts should not be undertaken when a neonate is born at this point in pregnancy." 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.Moore, Keith and Persaud, T. (2003). The Developing Human: Clinically Oriented Embryology\'. Philadelphia: Saunders, p. 103. ISBN 0-7216-9412-8.
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, 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 1987 in Ottawa, Canada, at 21 weeks and 5 days gestational age),Powell\'s Books - Guinness World Records 2004 (Guinness Book of Records) by. Retrieved on 2007-11-28.Miracle child. Retrieved on 2007-11-28. and Amillia Taylor (born on 24 October 2006 in Miami, Florida, at 21 weeks and 6 days gestational age).trithuc.thanhnienkhcn.org.vn. Retrieved on 2007-11-28."Most-premature baby allowed home", BBC News, 2007-02-21. Retrieved on 2007-05-05. Baptist Hospital of Miami, 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." Lee, Susan (August 24/31, 2005). "Fetal Pain A Systematic Multidisciplinary Review of the Evidence". The Journal of the American Medical Association 294 (8). the American Medical Association. Retrieved on 2008-02-14. 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, “Study Authors Didn\'t Report Abortion Ties”, New York Times (2005-08-26)."Study: Fetus feels no pain until third trimester" MSNBC 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. 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 pain and to suffer is part of the abortion debate.White, R. Frank. "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. "Recovering Damages for Fetal Pain and Suffering", Illinois Bar Journal (December 2002). Retrieved 2007-03-10. For example, 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.Weisman, Jonathan. "House to Consider Abortion Anesthesia Bill", Washington Post 2006-12-05. Retrieved 2007-02-06.
Fetal movement refers to motion of a fetus caused by its own muscle activity. Locomotor activity begins during the late embryological 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 self-generated nerve impulses originating in the spinal cord. As the nervous system matures, muscles can move in response to stimuli.Vaughan 1996, p. 208. Although the heart begins to beat on the 23rd day after conception, this is not considered to be a voluntary or reflex movement.
Some sources contend that there is no voluntary movement until after birth.White, Lois. Foundations Of Maternal & Pediatric Nursing, pages 10 and 128 (2004): "By the end of the 12th week, skeletal muscles begin involuntary movements....The newborn may cry and have muscular activity when cold, but there is no voluntary control of muscular activity." Other sources say that purposive movement begins months earlier."Insights into Early Fetal Development: "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".
Quickening is the first maternally discernable 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.Van Der Ziel, Cornelia & Tourville, Jacqueline. Big, Beautiful & Pregnant: Expert Advice And Comforting Wisdom for the Expecting Plus-size Woman (Marlowe 2006). Retrieved 2007-02-15.
Even before quickening, there is fetal movement. In addition to sideward bendings of the head, complex and generalized movements occur at the beginning of the fetal stage, with movements and startles that involve the whole body.Prechtl, Heinz. "Prenatal and Early Postnatal Development of Human Motor Behavior" in Handbook of brain and behaviour in human development, Kalverboer and Gramsbergen eds., pp. 415-418 (2001 Kluwer Academic Publishers): "The first movements to occur are sideward bendings of the head....At 9-10 weeks postmestrual age complex and generalized movements occur. These are the so-called general movements (Prechtl et al., 1979) and the startles. Both include the whole body, but the general movements are slower and have a complex sequence of involved body parts, while the startle is a quick, phasic movement of all limbs and trunk and neck." Movement of hands, hips and knees have been observed at nine weeks,Valman, H. and Pearson, J. "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." stretches and yawns at ten weeks,Butterworth, George and Harris, Margaret. Principles of developmental psychology, page 48 (Psychology Press 1994): "stretch and yawn pattern at 10 weeks." and isolated limb movements beginning shortly thereafter.Prechtl, Heinz. "Prenatal and Early Postnatal Development of Human Motor Behavior" in Handbook of brain and behaviour in human development, Kalverboer and Gramsbergen eds., pp. 415-418 (2001 Kluwer Academic Publishers): "At 9-10 weeks postmenstrual age complex and generalized movements occur... [I]solated movements of one arm or leg emerge 1 week later." 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.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
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.Whitaker, Kent. Comprehensive Perinatal and Pediatric Respiratory Care (Delmar 2001). Retrieved 2007-03-04.
Blood from the placenta is carried to the fetus by the umbilical vein. About half of this enters the fetal ductus venosus and is carried to the inferior vena cava, while the other half enters the liver proper from the inferior border of the liver. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the portal vein. The blood then moves to the right atrium of the heart. In the fetus, there is an opening between the right and left atrium (the foramen ovale), and most of the blood flows from the right into the left atrium, thus bypassing pulmonary circulation. The majority of blood flow is into the left ventricle from where it is pumped through the aorta into the body. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the woman\'s circulation.
Some of the blood from the right atrium does not enter the left atrium, but enters the right ventricle and is pumped into the pulmonary artery. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ductus arteriosus, which directs most of this blood away from the lungs (which aren\'t being used for respiration at this point as the fetus is suspended in amniotic fluid).
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.
Remnants of the fetal circulation can be found in adults:Dudek, Ronald and Fix, James. Board Review Series Embryology (Lippincott 2004). Retrieved 2007-03-04.University of Michigan Medical School, Fetal Circulation and Changes at Birth. Retrieved 2007-03-04.
| Fetal | Adult |
|---|---|
| foramen ovale | fossa ovalis |
| ductus arteriosus | ligamentum arteriosum |
| extra-hepatic portion of the fetal left umbilical vein | ligamentum teres hepatis (the "round ligament of the liver"). |
| intra-hepatic portion of the fetal left umbilical vein (the ductus venosus) | ligamentum venosum |
| proximal portions of the fetal left and right umbilical arteries | umbilical branches of the internal iliac arteries |
| distal portions of the fetal left and right umbilical arteries | medial umbilical ligaments (urachus) |
In addition to differences in circulation, the developing fetus also employs a different type of oxygen transport molecule than adults (adults use adult hemoglobin). Fetal hemoglobin enhances the fetus\' ability to draw oxygen from the placenta. Its 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.
Selection from "Views of a Fetus in the Womb", a drawing by Leonardo da Vinci.
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 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.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). "Fetal Homicide". Retrieved January 19, 2007.
Abortion of a fetus is legal in many countries such as Australia, Canada, Mexico, UK and USA.
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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, 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.[citation needed]
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