What Is Placenta Previa, and Why Should I Be Concerned?

What You Need To Know About Placenta Previa, and How It's Diagnosed

Types of Placenta Previa
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Placenta previa, the implantation of the placenta that covers at least part of the cervix, occurs in about one in 200 pregnancies. It can manifest in one of three ways.

What Are the Three Types of Placenta Previa?

Complete previa is when the cervical os, the mouth of the uterus, is completely covered.

Partial previa is when just a portion of the cervix is covered by the placenta.

Marginal previa extends just to the edge of the cervix.

How Is Placenta Previa Diagnosed?

Diagnosis of 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 percent false positive diagnosis rate, usually due to the bladder being overly full. There is also a 7 percent false negative rate typically caused by missing the previa that is located behind the baby's head.

Aside from bleeding, other reasons to suspect a previa include:

  • premature contractions
  • abnormal lie (breech, transverse, etc.)
  • the uterus measuring larger than is normal according to how far along you are in your pregnancy

During second trimester ultrasounds, done in some places on a routine basis, you may find that it is easier to get a previa diagnosis. Typically at 16 weeks, the placenta takes up 25-50 percent of the surface area.

Also, the third trimester brings a growth of this lower uterine segment, which outflanks the growth of the placenta.

For these reasons, while 5 percent of pregnancies are diagnosed with complete previa during second trimester ultrasounds, 90 percent of them are typically resolved by full term. And while 45 percent of pregnancies are diagnosed with marginal previas, 95 percent will be resolved by full term.


How Serious Is This Condition?

True placenta previa at full 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
  • need for a cesarean delivery
  • increased risk of postpartum hemorrhage
  • increased risk of placenta accreta (this is when the placenta attaches directly to the uterine muscle)

Placenta previa, once diagnosed, usually necessitates 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, an immediate cesarean delivery is usually done, regardless of the length of the pregnancy.

Some marginal previas can be delivered without cesarean surgery, while other types of placenta previa preclude vaginal delivery.

Am I At Risk for Placenta Previa?

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

    Placenta previa can be a scary diagnosis for all involved. The period of time from the diagnosis to the delivery are often periods of great worry and fear. Luckily, there are support groups for those who have been diagnosed with placenta previa, and/or who are under bed rest orders.

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