How many weeks are in each trimester of pregnancy?

A typical pregnancy is divided into three trimesters. The first trimester lasts from week 1 to week 12, the second trimester spans from week 13 to week 26, and the third trimester runs from week 27 to the end of the pregnancy at around week 40. Therefore, each trimester contains approximately 13 weeks, though the exact number can vary slightly depending on the specific timing of the pregnancy.