What complication occurs when blood vessels lie over or near the entrance to the birth canal?

Vasa previa is a specific obstetric complication that arises when fetal blood vessels, typically those belonging to the placenta, traverse or are positioned near the cervical opening. This condition poses a significant risk during labor, as these vessels can rupture with the pressure during contractions or upon rupture of the membranes. If not identified early and managed appropriately, vasa previa can lead to significant fetal hemorrhage, which can be life-threatening for the fetus.

In contrast, the other options do not directly relate to the position of blood vessels in relation to the birth canal. Preterm labor involves contractions leading to cervical changes before 37 weeks gestation and isn't specifically associated with the location of blood vessels. Gestational diabetes is a metabolic issue affecting glucose tolerance during pregnancy and does not pertain to vascular positioning. Placenta accreta, while related to placental attachment issues, refers to the abnormal adherence of the placenta to the uterine wall rather than blood vessels near the birth canal.

Thus, vasa previa is linked directly to the complication of blood vessels being positioned near the birth canal, making it the correct response.