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Placenta Previa

How is Placenta Previa Diagnosed

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Types of Placenta Previa

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Wolters Kluwer Health, Inc.- Lippincott Williams & Wilkins. All rights reserved.

Placenta previa, the implantation of the placenta at least partially covering the cervix, occurs in about one in 200 pregnancies. There are actually three types of previa.

Complete previa where the cervical os, the mouth of the uterus, is completely covered. Partial previa where just a portion of the cervix is covered by the placenta. And the marginal previa that extends just to the edge of the cervix.

Diagnosing a previa is usually made when there is painless bleeding during the third trimester. If you are bleeding it is unwise to do a vaginal exam until an ultrasound has ruled out a placenta previa. However, there is a 10% false positive diagnosis rate, usually because of the bladder being over full. There is also a 7% false negative rate, typically caused from missing the previa that is located behind the baby's head.

Other reasons to suspect a previa would sometimes be premature contractions, abnormal lie (breech, transverse, etc.), or the uterus measuring larger than you should according to dates.

During second trimester ultrasounds, done in some places on a routine basis, will show that there are many more previas diagnosed at this stage. Typically at 16 weeks the placenta takes up 25-50% of the surface area. Also the third trimester brings a growth of this lower uterine segment, that out flanks the growth of the placenta. For these reasons, while 5% of pregnancies are diagnosed with complete previa in second trimester ultrasounds will see 90% of them resolved by term and while 45% of pregnancies are diagnosed with marginal previas will see 95% resolved at term. A follow up ultrasound will be done, and as noted above the vast majority of previas are not seen.

True placenta previa at term is very serious. Complications for the baby include:

  • Problems for the baby, secondary to acute blood loss
  • Intrauterine growth restriction (IUGR) due to poor placental perfusion
  • Increased incidence of congenital anomalies

Risks for the mother include:

  • Life-threatening hemorrhage
  • Cesarean delivery
  • Increased risk of postpartum hemorrhage
  • Increased risk placenta accreta (Placenta accreta is where the placenta attaches directly to the uterine muscle.)

Placenta previa, once diagnosed, will usually mean bed rest for the mother, frequently in the hospital. Depending on the gestational age steroid shots may be given to help mature the baby's lungs. If the bleeding cannot be controlled immediate cesarean delivery is usually done, regardless of the length of the pregnancy. Some marginal previas can be delivered without cesarean surgery, the other types of placenta previa preclude vaginal delivery.

There are a few predisposing factors. The following can increase your risk for placenta previa:

  • Advanced maternal age
  • Increased parity (number of pregnancies)
  • Previous uterine surgery, including cesarean section (regardless of incision type)

Placenta previa can be a very scary diagnosis for all involved. The period of time from the diagnosis to the delivery are often periods of great worry and fear. There are support groups for bedrested mothers and even some for mothers with placenta previa. They are available to help you through this period of time.

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