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. Rapid diagnosis and adequate management will determine the timing of delivery and outcomes. The principal causes associated with antepartum hemorrhage are:
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 cervical os, below the presenting part of the fetus. The incidence is one in 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. Risk factors include a previous pregnancy with placenta previa, number of 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).
Modern approach to the ultrasonographic assessment of a low-lying placenta.
Delineation of the location of the placental edge in relation to the cervical os is of paramount importance. The marginal sinus of the placenta should not be mistaken by the placental edge. However, a marginal sinus covering the internal os is clinically relevant due to the high risk of antepartum bleeding. In the case of complete placenta previa, the degree of overlap beyond the cervical os should be estimated. In the case of a low-lying placenta, the distance from the os should be documented. A simplified and clinically relevant classification is proposed:
Placenta previa (Fig. 31-2): when the placenta is covering or overlapping the cervical os. Complete previa (or central) defines a placenta that overlaps the internal os covering the anterior lower uterine segment.
Low-lying placenta: when the placental edge is within 2 cm of the cervical os
Normal placental location: when the placental edge is more than 2 cm away from the cervical os
A posterior placenta with the lower edge just crossing the internal os on transvaginal ultrasound.
In addition, the thickness of the placental edge should be assessed (see Fig. 31-3). The presence of a thick placental edge (>1 cm or 45°) is associated with a higher risk of ...