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A fetus (or foetus or fœtus) is a developing human, after the embryonic stage and before childbirth. The plural is fetuses, or sometimes feti. The fetal stage of prenatal development starts at the beginning of the 11th week in gestational age (the 9th week after fertilization),  when the major structures have formed. This stage lasts until birth.
Etymology and spelling variations
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. 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.
- Main article: Prenatal 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 11-17The fetal stage commences at the beginning of the 11th week. 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. The head makes up nearly half of the fetus' size. Breathing-like movement of the fetus is necessary for stimulation of lung development, rather than for obtaining oxygen.The heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation.
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. 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. 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. 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). Fetal skin is almost transparent. The first measurable signs of EEG movement occur in the 12th week.
Weeks 18-27The 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. Feelers the fetus developed earlier fall off and are absorbed into the womb. 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. By the end of the fifth month, the fetus is about 20 cm (8 inches).
Weeks 28-40The 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, which means that the fetus is considered sufficiently developed for life outside the uterus. It may be 48 to 53 cm (19 to 21 inches) in length, when born.
Variation in growth
- See also: 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, placental, or fetal.
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). 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 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.
During the past several decades, neonatal care has improved with advances in medical science, and therefore the limit of viability has moved earlier. 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), and Amillia Taylor (born on 24 October 2006 in Miami, Florida, at 21 weeks and 6 days gestational age). 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.
- Main article: 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." 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. 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.  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.
- Main article: Fetal movement
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 may be felt as early as 18 weeks. Mothers can begin to feel quickening anywhere between 18 and 24 weeks of gestation.
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. Control of movement is limited at birth, and purposeful voluntary movements develop in the first year after birth. 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 system matures, muscles can move in response to stimuli, though this is not a voluntary movement. 
- Main article: Fetal circulation
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.
- Main article: 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 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 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 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.
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
|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.
- See also: 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 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  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. 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.
Abortion of a fetus is legal in many countries such as Australia, Canada, 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.
- Neural development
- Fetal position
- Fetal rights
- Women's rights
- ↑ 1.0 1.1 1.2 Klossner, N. Jayne Introductory Maternity Nursing (2005): "The fetal stage is from the beginning of the 9th week after fertilization and continues until birth"
- ↑ The American Pregnancy Association
- ↑ MedicineNet.com: "The unborn offspring from the end of the 8th week after conception (when the major structures have formed) until birth." See also 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."
- ↑ Harper, Douglas. (2001). Online Etymology Dictionary. Retrieved 2007-01-20.
- ↑ The American Heritage Dictionary of the English Language, Fourth Edition. Retrieved 2007-01-22.
- ↑ Aronson, Jeff (July 1997). When I use a word...:Oe no!. British Medical Journal 315 (1).
- ↑ • 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.
- ↑ 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)."
- ↑ 9.0 9.1 9.2 MedlinePlus
- ↑ Institute of Medicine of the National Academies, Preterm Birth: Causes, Consequences, and Prevention (2006), page 317. Retrieved 2008-03-12
- ↑ The Columbia Encyclopedia (Sixth Edition). Retrieved 2007-03-05.
- ↑ Greenfield, Marjorie. “Dr. Spock.com". Retrieved 2007-01-20.
- ↑ 13.0 13.1 Lee, Susan (August 24/31, 2005). Fetal Pain A Systematic Multidisciplinary Review of the Evidence. The Journal of the American Medical Association 294 (8): 947. (see Fetal Pain section)
- ↑ 14.0 14.1 "Study: Fetus feels no pain until third trimester" MSNBC
- ↑ 15.0 15.1 15.2 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)
- ↑ Mayo Clinic
- ↑ 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.
- ↑ Levene, Malcolm et al. Essentials of Neonatal Medicine (Blackwell 2000), page 8. Retrieved 2007-03-04.
- ↑ Your Pregnancy: 36 Weeks BabyCenter.com Retrieved June 1 2007.
- ↑ "full-term" defined by Memidex/WordNet.
- ↑ 21.0 21.1 Holden, Chris and MacDonald, Anita. Nutrition and Child Health (Elsevier 2000). Retrieved 2007-03-04.
- ↑ Queenan, John. Management of High-Risk Pregnancy (Blackwell 1999). Retrieved 2007-03-04.
- ↑ 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."
- ↑ Moore, Keith and Persaud, T. (2003). The Developing Human: Clinically Oriented Embryology', p. 103, Philadelphia: Saunders.
- ↑ 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.
- ↑ Powell's Books - Guinness World Records 2004 (Guinness Book of Records) by. URL accessed on 2007-11-28.
- ↑ Miracle child. URL accessed on 2007-11-28.
- ↑ includeonly>"Most-premature baby allowed home", BBC News, 2007-02-21. Retrieved on 2007-05-05.
- ↑ Baptist Hospital of Miami, Fact Sheet (2006).
- ↑ 30.0 30.1 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.
- ↑ 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.
- ↑ Weisman, Jonathan. "House to Consider Abortion Anesthesia Bill", Washington Post 2006-12-05. Retrieved 2007-02-06.
- ↑ 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.
- ↑ 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.
- ↑ 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
- ↑ Stanley, Fiona et al. "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. "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)
- ↑ Vaughan 1996, p. 208.
- ↑ Valman, H. and Pearson, J. "What the Fetus Feels", British Medical Journal, (January 26 1980).
- ↑ Butterworth, George and Harris, Margaret. Principles of developmental psychology, page 48 (Psychology Press 1994): "stretch and yawn pattern at 10 weeks."
- ↑ 42.0 42.1 42.2 Whitaker, Kent. Comprehensive Perinatal and Pediatric Respiratory Care (Delmar 2001). Retrieved 2007-03-04.
- ↑ 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.
- ↑ Dalby, JT. (1978).Environmental effects on prenatal development Journal of Pediatric Psychology, 3, 105-109.
- ↑ Streissguth, A. (1997). Fetal Alcohol Syndrome: A Guide for Families and Communities. Baltimore: Brookes Publishing. ISBN 1-55766-283-5.
- ↑ National Conference of State Legislatures. (June 2006). "Fetal Homicide". Retrieved January 19 2007.
- ↑ Anika Rahman, Laura Katzive and Stanley K. Henshaw. A Global Review of Laws on Induced Abortion, 1985-1997, International Family Planning Perspectives (Volume 24, Number 2, June 1998).
|Stages of human development|
|Mammalian development of embryo and development and fetus (some dates are approximate - see Carnegie stages) - edit|
Week 3: Hensen's node | Gastrula/Gastrulation | Trilaminar embryo Branchial arch (1st) | Branchial pouch | Meckel's cartilage | Somite/Somitomere | Germ layer (Ectoderm, Endoderm, Mesoderm, Chordamesoderm, Paraxial mesoderm, Intermediate mesoderm, Lateral plate mesoderm)
|Histogenesis and Organogenesis|
Circulatory system: Primitive atrium | Primitive ventricle | Bulbus cordis | Truncus arteriosus | Ostium primum | Foramen ovale | Ductus venosus | Ductus arteriosus | Aortic arches | Septum primum | Septum secundum | Cardinal veins
Urinary/Reproductive system: Urogenital folds | Urethral groove | Urogenital sinus | Kidney development (Pronephros | Mesonephros | Ureteric bud | Metanephric blastema) | Fetal genital development (Wolffian duct | Müllerian duct | Gubernaculum | Labioscrotal folds)
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