What is placenta previa?
The placenta normally lies high in the uterus near the top. With placenta previa, the placenta lies low in the uterus and covers or partially covers the inner opening of the cervix. This can cause hemorrhaging before or during delivery which can be serious for both mom and baby.
There are three types of placenta previa:
- marginal - the placenta touches only the edge of the cervix
- partial - the placenta partially covers the cervix
- complete - the cervix is completely covered by the placenta
When the placenta is located near the cervix without touching the cervix, it's termed a low-lying placenta.
Placenta previa occurs in approximately 1 out of 200 births. The recurrence rate for women who have had a previous placenta previa is 4 to 8%. It's rare for first-time mothers under 30.
What are the causes of placenta previa?
The cause of placenta previa is unknown although there are some risk factors. The condition occurs more often in women:
- who have had previous pregnancies
- who smoke
- who are older
- who have had a previous cesarean birth or other uterine surgery
- who are pregnant with multiples
What are the symptoms of placenta previa?
The most common symptom is painless bright red bleeding. Placenta previa is the most common cause of vaginal bleeding in the second and third trimesters. Most "first bleeds" happen in the last second trimester or early third trimester.
Abnormal lie of the baby can arouse suspicion of placenta previa. Of all transverse lies, 60% are associated with placenta previa, and of all breech and compound lies, 24% are associates with placenta previa.1
How do I know if I have placenta previa?
Many women learn they have placenta previa or a low-lying placenta during a routine ultrasound done in the mid-second trimester. They have experienced no symptoms and wouldn't have known except for the ultrasound. In other cases, an ultrasound is performed due to bleeding. In 90% of the women diagnosed around 16-20 weeks, the placenta migrates out of the way by 30 weeks and is not longer a concern.
What are the risks of placenta previa?
Placenta previa can cause hemorrhaging either before birth (leading to a preterm birth) or during delivery. Except for some marginal previas, delivery is nearly always via cesarean section which has the typical surgical risks.
Placenta previa may be associated with abnormal attachment of the placenta to the uterus placenta accreta, placenta increta, and placenta percreta), especially if the placenta is located over a previous cesarean section scar.2
What is the treatment for placenta previa?
If placenta previa has been diagnosed via ultrasound in the second trimester and there is no bleeding, avoidance of strenuous activity and a follow up ultrasound around 28-30 weeks will probably be recommended. If placenta previa is still present at 30 weeks, activities may be further restricted and bedrest may be prescribed.
If bleeding is present, the care provider should be notified immediately. Bedrest and/or hospitalization is often necessary. If hemorrhaging develops, the baby may need to be delivered to save mom's life. If the bleeding can be controlled and the baby is less than 36-37 weeks gestation, steroid shots may be given to help mature the baby's lungs.
Placenta previa can be a frightening complication but with monitoring and care, it doesn't necessarily mean a bad outcome for mom or baby. Remember that 90% of all placenta previas diagnosed in the second trimester are NOT present by 30 weeks.
1. Heppard, Martha C.S. and Thomas J. Garite. Acute Obstetrics (Missouri: Mosby-Year Book, Inc, 1996), pp 215.
2. Lima, David M, MD. Placenta Causes of Fetal Loss. Hygeia, Vol 1, No 7. 2002.
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