The low-down on down low placentas: Placenta previa

It's midnight and a first-time mother in her second trimester presents to the emergency room with vaginal bleeding. What could it be? Hopefully, it's something relatively benign like a friable cervix, but we always keep in mind that vaginal bleeding can be a sign of an abnormality with the placenta. The September 2015 Green Journal includes a Clinical Expert Series on Abnormal Placentation by Dr. Silver of The Utah School of Medicine in Salt Lake City. He reviews three types of abnormalities we can see with placentas: placenta previa, vasa previa, and placenta accreta. Here is a run-down of the highlights from this informative review on one type of abnormal placental location: placenta previa.

We call it placenta previa when the placenta lies over the baby's exit route, the cervical os. With advanced ultrasounds, we can now identify where the placenta is in relation to the cervical opening. We characterize placentas as "placenta previa" when it lies over the os and "low lying placenta" when it is near the os. A good cut off for how far the placenta should be from the os before worrying about placenta previa is 4.2cm. Fortunately, only 10-20% of placenta previas diagnosed in the second trimester will still be previas by the end of pregnancy. For this reason, re-evaluation of placenta previas with ultrasound, usually around 32 weeks, is necessary. Risk factors include things that cause endometrial damage: prior cesarean section, uterine scar from another type of surgery, prior spontaneous or elective pregnancy termination, and increasing number of prior births. It occurs in about 1 in 200 pregnancies.

Women with placenta previa have a 10x risk of vaginal bleeding during pregnancy. If a woman has one episode of bleeding and then it stabilizes, it is reasonable to follow her in the outpatient setting. However, most experts agree that women with multiple episodes of bleeding should remain in the hospital until delivery. It is controversial whether placing a cerclage to stabilize the cervix is helpful, and the existing evidence conflicts. Women with placenta previa must have a c-section for delivery, usually during the 36th week of gestation. As such, about 44% of women with placenta previa will deliver preterm. If the placenta is 2cm or further from the os, it is reasonable to attempt a vaginal delivery.

I highly recommend this informative and succinct review of placentas to learn more about vasa previa and placenta accreta.