Hemorrhage in the second half of pregnancy can pose a serious threat to the health of both mother and child. In the majority of cases, the exact cause will remain unknown antenatally.
Local lesions (e.g., cervical ectropion or polyp)
Unknown or idiopathic: no discoverable cause
In this condition, the placenta is implanted in the lower uterine segment and lies over or near the internal os of the cervix. It is below the presenting part of the fetus. The incidence is 1:350 pregnancies. It is responsible for about 10 percent of antepartum hemorrhages. The late development of the lower uterine segment after 28 weeks leads to the phenomenon of placental migration, in which an apparent placenta previa in early pregnancy moves away from the internal os toward term.
The etiology is unknown. Epidemiologic associations include a previous pregnancy with placenta previa, previous cesarean sections, and advanced maternal age. Weaker risk factors include multiparity, endometrial trauma such as curettage, and cigarette smoking.
The clinical classification of placenta previa is now based on the ultrasound findings (Fig. 31-1). Because of the possibility of placental migration, final decisions regarding mode of delivery in asymptomatic patients should only be based on the ultrasound appearance at 35+ weeks. Before then, its appearance should only guide decision making if the patient labors preterm, although a placenta that overlaps the internal os by more than 20 mm is associated with a low chance of migration.
Modern approach to the ultrasonographic assessment of a low-lying placenta.
Lower edge of placenta >20 mm from internal os: The risk of significant intrapartum bleeding necessitating delivery by cesarean section is low, and vaginal delivery can be attempted
Lower edge of placenta within 20 mm of the internal os but does not overlap it: Studies have shown that some women will have successful vaginal births in this situation but that the incidence of emergency cesarean section is high (40%-90%). Intrapartum management should be dictated by individual clinical circumstances and local resources
The lower edge of placenta overlaps the internal os by any amount (Fig. 31-2): Delivery should be by cesarean section
A posterior placenta with the lower edge just crossing the internal os on transvaginal ultrasound.
The classic symptom is painless vaginal bleeding. A feature of placenta previa is that the degree of anemia or shock is equivalent to the amount of blood lost. In most cases, the bleeding is unprovoked, but ...