Which are the primary causes of third-trimester bleeding?

The primary causes of third-trimester bleeding include placental abruption and placenta previa, which are critical conditions that require immediate medical evaluation and management due to their potential complications for both the mother and the fetus.

Placental abruption occurs when the placenta detaches from the uterine wall prematurely, which can lead to significant maternal bleeding and fetal distress or demise. It is often associated with abdominal pain, back pain, and sometimes uterine rigidity, in addition to the bleeding.

Placenta previa, on the other hand, happens when the placenta implants low in the uterus, covering part or all of the cervix. This can lead to painless vaginal bleeding, particularly as the cervix begins to efface or dilate in preparation for labor. Diagnosis is typically made via ultrasound, and management often involves delivery planning depending on the gestational age and extent of the previa.

Other options mentioned involve conditions that are not as commonly associated with third-trimester bleeding or do not directly lead to significant vaginal bleeding. Preterm labor and cervical insufficiency, for instance, primarily involve contractions and cervical changes rather than bleeding. Vasa previa and uterine atony can lead to bleeding but are less common and typically are classified under different mechanisms of third-trimester complications