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Viability
Stages in prenatal development, showing viability and point of 50% chance of survival at bottom. Weeks and months numbered by gestation.
The lower limit of viability is approximately five months gestational age, and usually later.[31]
About 20 to 35 percent of babies born at 23 weeks of pregnancy survive, while about 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive
Human fetus, age unknown
According to The Developing Human, viability is defined as the ability of fetuses to survive in the extrauterine environment.[32]
There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. [32] 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.[33] It is rare for a baby weighing less than 500 gm to survive.[32]
When such babies are born, the main causes of perinatal mortality is that the respiratory system and the central nervous system are not completely differentiated.[32] If given expert postnatal care, some fetuses weighing less than 500 gm may survive, being are referred to as extremely low birth weight or immature infants.[32] Preterm birth is the most common cause of perinatal mortality, causing almost 30 percent of neonatal deaths.[34]
During the past several decades, neonatal care has improved with advances in medical science, and therefore the limit of viability has moved earlier.[35] 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),[36][37] and Amillia Taylor (born on 24 October 2006 in Miami, Florida, at 21 weeks and 6 days gestational age).[38][39] 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.
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