The placenta has been hard at work throughout your pregnancy, transporting nutrients and oxygen from your blood to your baby via the umbilical cord. Normally this organ is attached high on the wall of your uterus; problems may occur if instead the placenta grows over the cervix, a condition called placenta previa.
Normally during childbirth, the baby arrives before the placenta, traveling through your cervix and vagina. Within half an hour of your baby's birth, you will deliver the placenta when it separates from the wall of your uterus. This sequence of events is necessary because your baby relies on the placenta to provide life-sustaining oxygen until he can breathe on his own.
But if the placenta grows low in the uterus, it may partly or completely block the opening in the cervix that leads to your vagina. This can result in heavy bleeding during labor and delivery, which can be dangerous for mother and baby.
Placenta previa happens in about 1 in 200 pregnancies. The cause of placenta previa is unknown, but like placental abruption, a condition in which the placenta separates from the uterine wall, it's more common among women who smoke, use cocaine, or are over age 35.
Women are also at increased risk if they've had previous uterine surgery, including a c-section, a D&C (dilation and curettage, in which the lining of the uterus is scraped) following a miscarriage, an abortion, or if they are carrying twins (or a higher number of multiples).
Complete Previa: The cervical opening is completely covered by the placenta
Partial Previa: A portion of the cervix is covered by the placenta
Marginal Previa: The placenta extends just to the edge of the cervix
The biggest concern of placenta previa is that it can heighten the risk of life-threatening hemorrhage, either before or during labor. This severe, uncontrollable bleeding can happen because, as your cervix thins out and opens in preparation for delivery, the placenta's attachments to the uterus can more easily become damaged due to its lowered position. If this happens you might start bleeding without any pain at all.
Your provider will do an ultrasound to determine the cause of the bleeding. An ultrasound at 18-20 weeks may show a low-lying placenta or a placenta previa. The ultrasound will be repeated in the 3rd trimester, when most low-lying placentas will no longer be low because the uterus has grown larger. If the placenta still completely covers the cervix at the time of the 3rd trimester ultrasound, most of the time it stays there. Placenta previa is usually diagnosed during the last 2 months of pregnancy.
Placenta previa treatment goals are to prevent bleeding and lessen the risk of premature delivery.
More than 90 percent of the time, placenta previa diagnosed in the second trimester corrects itself by term. You don't need to restrict your activities or undergo any treatment. Your doctor will probably recommend another ultrasound at about 28 weeks, though, to make sure the placenta has moved away from the cervix.
In the unlikely event that placenta previa is diagnosed during the third trimester but there is no bleeding, your provider will probably tell you to go on bed rest or limit your activities to lessen the danger of bleeding until your baby is big enough to safely deliver by cesarean. Cesarean deliveries are almost always necessary because the placenta would be torn from its roots during a vaginal delivery, causing life-threatening bleeding for mom and lack of oxygen for baby.
If you start to bleed during your third trimester, you will be admitted to the hospital; how long you remain in the hospital depends on several factors. Your doctor will do an ultrasound exam to diagnose placenta previa and pinpoint the placenta's location. If the bleeding stops, which it usually does, your physician will continue to monitor both you and baby. If the bleeding does not stop, or if you go into labor, your doctor will suggest a c-section.
If you haven't yet gone into labor but your doctor thinks you may deliver before 34 weeks, she'll probably recommend treatment with corticosteroids. At 36 weeks, if you haven't delivered, she may also suggest a test called amniocentesis to see if your baby's lungs are mature. Provided they are, she will likely suggest a c-section to prevent serious uterine bleeding.