Placenta Previa

Types of placenta previa

Older terms such as "marginal" and "partial" previa are now grouped into these two categories.

Can placenta previa correct itself?

Often, yes. The placenta does not actually migrate. As the lower part of the uterus stretches and grows during the second and third trimesters, the placenta's position relative to the cervix shifts upward. This is sometimes called "placental migration."

• Most low-lying placentas found at the 18 to 22 week scan resolve by the third trimester.
• Complete previa, especially one that covers the cervix substantially, and previas on the back wall of the uterus or over a cesarean scar are less likely to resolve.
• A follow-up transvaginal ultrasound around 32 weeks checks the position. If still low, another scan is often done around 36 weeks.

Transvaginal ultrasound is safe with previa and is the most accurate way to measure the distance between the placenta and the cervix.

Symptoms

• sudden, painless, bright red vaginal bleeding, often in the late second or third trimester; it may stop and recur
• bleeding after sex or a vaginal exam
• sometimes contractions with bleeding
• many people have no symptoms; the previa is found on routine ultrasound

Painful bleeding with a tense, tender uterus suggests placental abruption instead.

Risk factors

• previous cesarean delivery or uterine surgery (such as fibroid removal)
• previous placenta previa
• multiple pregnancy (twins or more)
• IVF pregnancy
• maternal age over 35
• smoking and cocaine use
• having had several pregnancies

Placenta accreta

When previa occurs in someone with a previous cesarean, there is a higher risk of placenta accreta spectrum, where the placenta grows too deeply into the uterine wall. This can cause life-threatening bleeding at delivery and requires planned delivery at a specialist center, sometimes with hysterectomy. Detailed ultrasound or MRI is used to assess for accreta.

Management

During pregnancy

• avoid vaginal exams by hand; only ultrasound or speculum exams
• pelvic rest: your doctor may advise avoiding sex and inserting anything into the vagina, particularly if you have bled
• know where to go: report any bleeding immediately; live within reach of a hospital with blood transfusion and cesarean facilities
• steroid injections to mature the baby's lungs if early delivery seems likely
• anti-D injection if you are Rh negative and bleed
• iron if anemic
• hospital admission after significant bleeding, sometimes until delivery

Routine strict bed rest is not recommended.

Delivery

• If the placenta still covers or is within 2 cm of the cervix late in pregnancy, a planned cesarean delivery is recommended, usually around 36 to 37 weeks for uncomplicated previa.
• Emergency cesarean is needed for heavy bleeding, labor, or concerns about the baby.
• When the placenta edge is more than 2 cm away, vaginal birth may be possible.

Complications

• heavy bleeding before, during or after delivery (postpartum hemorrhage)
• preterm birth
• placenta accreta spectrum
• need for blood transfusion or hysterectomy
• vasa previa, where fetal blood vessels run near the cervix (rare)

Frequently asked questions

Can placenta previa correct itself?

Yes, in most cases. As the uterus grows, a low-lying placenta often ends up well away from the cervix. Around 9 in 10 low placentas found at the mid-pregnancy scan have resolved by the third trimester. A follow-up scan around 32 weeks confirms its position.

What are the symptoms of placenta previa?

The main symptom is sudden, painless, bright red vaginal bleeding in the second half of pregnancy. Some people have no symptoms at all, and the previa is found on a routine ultrasound.

Can you have a vaginal birth with placenta previa?

Not if the placenta still covers the cervix or lies within 2 cm of it near term. A planned cesarean is recommended. If the placenta has moved further away, vaginal birth may be possible.

What should I avoid with a low-lying placenta?

Your doctor may advise avoiding sex, heavy lifting and strenuous exercise, especially if you have had bleeding. Avoid vaginal examinations by hand. Report any bleeding immediately.