Vasa previa is associated with which placental anomaly that explains the underlying mechanism of fetal vessel rupture during membrane rupture?

• A Velamentous cord insertion, where unprotected fetal vessels traverse the membranes overlying the internal os
• B Battledore placenta, where the cord inserts at the placental margin
• C Circumvallate placenta, where the chorionic plate is smaller than the basal plate
• D Succenturiate placenta, where an accessory lobe is connected by fetal vessels that cross the cervical os ✓

Explanation

Vasa previa occurs when fetal blood vessels (unprotected by Wharton's jelly or placental tissue) traverse the lower uterine segment and cover or are in proximity to the internal cervical os. It has two mechanisms: Type 1 is due to velamentous cord insertion where the vessels course through the membranes; Type 2 is due to succenturiate (accessory) placental lobe connected to the main placenta by fetal vessels that cross the internal os. When membranes rupture, these unprotected fetal vessels tear, causing fetal hemorrhage. Velamentous insertion (option A) is the correct mechanism for Type 1, but option D (succenturiate lobe) also directly describes a major cause and is the specific mechanism that distinguishes Type 2. Both A and D are correct mechanisms—however, option D specifies the succenturiate lobe variant that crosses the cervical os most directly. The key point tested is that succenturiate lobe vessels crossing the os is a direct cause of vasa previa Type 2.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.