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Viability

Fetal viability refers to a point in fetal development at which the fetus may survive outside the womb. The lower limit of viability is approximately 5 + 3 ⁄ 4 months gestational age and is usually later.

There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. According to data from 2003 to 2005, survival rates are 20–35% for babies born at 23 weeks of gestation (5 + 3 ⁄ 4 months); 50–70% at 24–25 weeks (6 – 6 + 1 ⁄ 4 months); and >90% at 26–27 weeks (6 + 1 ⁄ 2 – 6 + 3 ⁄ 4 months) and over. It is rare for a baby weighing less than 500 g (1 lb 2 oz) to survive.

When such premature babies are born, the main causes of mortality are that the respiratory system and the central nervous system are not completely differentiated. If given expert postnatal care, some preterm babies weighing less than 500 g (1 lb 2 oz) may survive, and are referred to as extremely low birth weight or immature infants.

Preterm birth is the most common cause of infant mortality, causing almost 30 percent of neonatal deaths. At an occurrence rate of 5% to 18% of all deliveries, it is also more common than postmature birth, which occurs in 3% to 12% of pregnancies.