What is the most common cause of third trimester bleeding and when does it occur?

The most common cause of third-trimester bleeding is abruptio placenta, which typically occurs after 20 weeks of gestation, often in the last trimester. Abruptio placenta refers to the premature separation of the placenta from the uterine wall, which can lead to significant maternal and fetal complications. This condition is particularly concerning because it can result in maternal hemorrhage and fetal distress or death if not managed promptly.

In contrast, placenta previa usually presents with bleeding that occurs before 20 weeks, and it is caused by the placenta being positioned over or near the cervix, which can also lead to complications but is less common compared to abruptio placenta in the third trimester. Cervical polyps are generally benign and would not commonly cause significant bleeding intrapartum or in the third trimester. Uterine rupture is a critical situation that occurs mainly at term, particularly in women with a history of cesarean sections, but it is less prevalent compared to abruptio placenta as a cause of bleeding. Thus, the primary association with third-trimester bleeding is with abruptio placenta occurring after 20 weeks of gestation.