The age of viability refers to the period of time when the developing fetus is able to survive outside of its mother’s womb. Generally, the age of viability of a fetus is considered to be between 22 and 26 weeks after conception. Survival of a child born prior to the full gestational age (40 weeks) improves greatly during the third trimester. The survival rates at different times during the age of variability are as follows: 21 weeks or less: survival rate is 0%.