Within both health care and political debate, the "age of viability," at which a fetus obtains the capacity to survive independently of its mother outside the womb. There is no specific scientific or ethical consensus regarding the exact age of viability for a fetus. Depending on how one defines "viable," a fetus may pass this milestone any time between 21 weeks and 28 weeks of gestation.
Because there are constant advances in care for preterm babies, the definiton of viability has changed dramatically in recent years. In the past, babies born at 30 weeks usually died. However, advances in medicine have enabled a few newborns to survive even if they are born as early as 21 weeks into gestation.
Earliest Age of Viability
The most preterm baby ever to survive outside the womb was Amillia Taylor, who was born at 21 weeks and 6 days of gestation. Taylor was born weighing less than ten ounces and measuring about the same length as a ballpoint pen. Based on her case, some have suggested that a 21-week-old fetus is viable. However, the odds of a baby's survival at this age are currently less than one-half of one percent.
Early Age of Viability
Some practitioners quote 23 weeks as the age of viability for a fetus, although babies born at this stage of gestation are still more likely to die than survive. According to data from 2003-2005, preterm babies born at 23 weeks have a 20-30% chance of survival if they receive good medical care. Based on these odds of survival, many practitioners regard this as the earliest plausible definition of the age of viability.
Median Age of Viability
With medical care, a fetus has a 50-70% chance of survival if it is born at 24 to 25 weeks gestation. If you define viability as a 50% chance of survival, a fetus reaches viability at about 24 weeks. However, newborns who are this significantly premature need excellent medical and maternal care to maximize their chances of survival. Babies born at 24 weeks were not considered viable 100 years ago, when their chances of survival were minimal.
Late Age of Viability
A fetus has a very good chance for survival outside the womb if it is born at 26 to 27 weeks gestation. With medical care, babies born at this stage have a survival rate of slightly more than 90%. Since the majority of these newborns survive, this is considered to be the most conservative estimate of a fetus's age of viability. Nevertheless, these babies do need several days or weeks of intensive treatment to help them survive.
Latest Age of Viability
Babies born after 30 weeks gestation have a more than 95% chance of survival, but they are still likely to have some complications any time before 37 weeks. A baby is viable, but not full-term, if it is between 30 and 37 weeks of gestation. The age of viability also changes for twins, small-for-gestational age babies, babies with congenital defects, and babies born in areas without adequate medical care. If you are pregnant and have any questions about your unborn child's viability, talk to your doctor or midwife.
This chart, based on information collected from 2003-2005, illustrates the survival rates of preterm babies.