Fetal viability is the ability of a fetus to survive outside the uterus.[1]
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There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable.[1] According to data years 2003-2005, 20 to 35 percent of babies born at 23 weeks of gestation survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive.[2] It is rare for a baby weighing less than 500g (17.6 ounces) to survive.[1]
The United States Supreme Court stated in Roe v. Wade (1973) that viability (i.e., the "interim point at which the fetus becomes ... potentially able to live outside the mother's womb, albeit with artificial aid"[3]) "is usually placed at about seven months (28 weeks) but may occur earlier, even at 24 weeks."[3] The 28-week definition became part of the "trimester framework" marking the point at which the "compelling state interest" (under the doctrine of strict scrutiny) in preserving potential life became possibly controlling, permitting states to freely regulate and even ban abortion after the 28th week.[3] The subsequent Planned Parenthood v. Casey (1992) modified the "trimester framework," permitting the states to regulate abortion in ways not posing an "undue burden" on the right of the mother to an abortion at any point before and after viability; on account of technological developments between 1973 and 1992, viability itself was legally dissociated from the hard line of 28 weeks, leaving the point at which "undue burdens" were permissible variable depending on the technology of the time and the judgment of the state legislatures.
The limit of viability is the gestational age at which a prematurely born fetus/infant has a 50% chance of long-term survival outside its mother's womb. With the support of neonatal intensive care units, the limit of viability in the developed world has declined since 50 years ago, but has remained unchanged in the last 12 years.[4][5] Currently the limit of viability is considered to be around 24 weeks although the incidence of major disabilities remains high at this point.[6][7] Neonatologists generally would not provide intensive care at 23 weeks, but would from 26 weeks.[8][9]
During the past several decades, neonatal care has improved with advances in medical science, and therefore the limit of viability has moved earlier.[10] 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),[11][12] and Amillia Taylor (born on 24 October 2006 in Miami, Florida, at 21 weeks and 6 days gestational age).[13][14] Both children were born just under 22 weeks from fertilization, or a few days past the midpoint of an average full-term pregnancy.
Amillia Taylor is also often cited as the most-premature baby.[15] She was born on 24 October 2006 in Miami, Florida, at 21 weeks and 6 days gestation.[16] This report has created some confusion as her gestation was measured from the date of conception (through in-vitro fertilization) rather than the date of her mother's last menstrual period making her appear 2 weeks younger than if gestation was calculated by the more common method.[17] At birth, she was 9 inches (22.86 cm) long and weighed 10 ounces (283 grams).[15] She suffered digestive and respiratory problems, together with a brain hemorrhage. She was discharged from the Baptist Children's Hospital on 20 February 2007.[15]
The lower limit of viability is approximately five months gestational age, and usually later.[18]
When preterm babies are born, the main causes of perinatal mortality is that the respiratory system and the central nervous system are not completely differentiated,[1] causing infant respiratory distress syndrome. "If given expert postnatal care, some fetuses weighing less than 500g may survive; they are referred to as extremely low birth weight or immature infants."[1] Preterm birth is the most common cause of perinatal mortality, causing almost 30 percent of neonatal deaths.[19]