The viability week in pregnancy is generally considered to be around 23 to 24 weeks of gestation. This mark indicates when a fetus has a potential to survive outside the uterus, often with the help of advanced medical support such as a neonatal intensive care unit (NICU). Survival rates increase significantly starting at this time—with survival rates around 23-27% at 23 weeks, about 42-59% at 24 weeks, and 67-76% at 25 weeks. Before 23 weeks, survival is very low (5-6%), and the chances improve as the fetus matures further past 24 weeks. Viability is not absolute and depends on multiple factors, including fetal weight, anatomy, genetics, lung development, and the availability of medical technology and care. It is considered the threshold where a fetus can potentially survive outside the womb, but not guaranteed survival. In summary:

• Viability is commonly defined as starting at 23 to 24 weeks gestation.
• Survival chances improve dramatically after 24 weeks.
• Births before 23 weeks are considered periviable with very low survival rates.
• Medical advancements have gradually shifted this viability threshold earlier over time.

This means that the viability week in pregnancy is around week 23 or 24.