A full-term pregnancy lasts approximately 40 weeks (280 days) from the first day of the last menstrual period, though healthy pregnancies typically range from 37 to 42 weeks. Babies born before 37 weeks are considered premature, while those born after 42 weeks are considered post-term.
The fetus can survive in the womb well beyond 40 weeks if the pregnancy continues, but extended pregnancies carry increasing risks. After 42 weeks, the placenta may become less efficient at delivering oxygen and nutrients, and complications like stillbirth become more likely. For this reason, most medical providers recommend inducing labor by 42 weeks to reduce these risks.
The earliest point at which a baby has a reasonable chance of surviving outside the womb is around 22-24 weeks of pregnancy, though survival at this stage requires intensive neonatal care and comes with significant health challenges. Viability improves substantially after 28 weeks, and outcomes become much more favorable after 32-34 weeks.
On the other end, some pregnancies are intentionally continued past 40 weeks if both mother and baby are healthy, though this is done under close medical monitoring. The key factor is that the womb provides optimal conditions for fetal development, but those conditions become less adequate as pregnancy extends beyond term, making timely delivery important for the baby's wellbeing.