Final answer:
A woman who has not had a birth at more than 20 weeks gestation is termed a 'nullipara'. Fetal viability is the potential for a fetus to survive outside the uterus, usually achievable around 24 to 26 weeks of gestation.
Step-by-step explanation:
The term that means a woman has not had a birth at more than 20 weeks gestation is 'nullipara'. This term is used to describe a woman who has never given birth to a viable child, and it is a classification used in obstetrics. A viable child is one that has reached the fetal viability stage, which is the point in fetal development at which the fetus is likely to be able to survive outside the uterus. The gestation period of a human pregnancy is typically around 40 weeks, and the threshold for fetal viability is generally considered to be around 24 to 26 weeks of gestation.