Final answer:
Abruptio placenta most commonly occurs in the third trimester of pregnancy, which lasts from weeks 29 through birth.
Step-by-step explanation:
Abruptio placenta, which refers to the premature separation of the placenta from the uterus, most commonly occurs in the third trimester of pregnancy. The third trimester spans from weeks 29 through birth, around 40 weeks. During this time, the fetus undergoes significant growth and the uterus expands rapidly, which may contribute to conditions like abruptio placenta. While the placenta takes over more functions as the baby develops during the second trimester, it is the third trimester where the larger size of the fetus and uterus can lead to complications such as abruptio placenta, characterized by bleeding and potential risk to both mother and fetus.