A preterm baby is born before which completed week of gestation?

The correct answer is based on the medical definition of prematurity, which is classified as the birth of a baby before 37 completed weeks of gestation. A full-term pregnancy is typically around 39 to 40 weeks, and anything less than 37 weeks is considered preterm.

Choosing 37 weeks as the cutoff aligns with established medical guidelines, such as those from the World Health Organization and the American College of Obstetricians and Gynecologists. Babies born between 37 and 38 weeks are often classified as "early term," which is different from preterm.

Therefore, the definition and classification of preterm birth focus on the 37-week mark, substantiating why it is the correct answer in this context.