If the placenta partially or entirely blocks the regular vaginal delivery route, a doctor may refer to this as placenta previa.

The placenta is an organ that develops inside the uterus during pregnancy. It provides oxygen and nutrients to the developing fetus.

Placenta previa involves the placenta either partly or completely covering the opening of the uterus into the cervix, which is called the internal os.

The exact position of the placenta will determine to what extent placenta previa affects pregnancy and limits the options for delivery.

Placenta previa affects around 3–5 births out of every 1,000. Below, learn about the types, symptoms, and risks of this issue and how doctors identify it.

a woman undergoing an ultrasound on her stomach to see if she has a Placenta previaShare on Pinterest
A doctor may identify placenta previa during a routine ultrasound.

Doctors classify placenta previa according to the position of the placenta. The four main types are:

  • low-lying
  • marginal
  • partial
  • complete

It is important for doctors to identify the position of the placenta, as doing so can help them reduce associated risks.


Almost 90% of low-lying placentas realign, shifting into the right place, by the third trimester.

A low-lying placenta is between 2.5 centimeters (cm) and 3.5 cm away from the internal os — the opening of the uterus into the cervix.


A marginal placenta borders the opening but does not obstruct it. The edge of the placenta is within 2 cm of the internal os.


This is the less severe type of placenta previa. It involves the placenta slightly covering internal os.

Partial placenta previa often resolves on its own during pregnancy. If it does not, it may still be possible to deliver vaginally.


Complete placenta previa involves the placenta covering the entire internal os.

This type has the greatest implications, and it means that the woman will have to undergo cesarean delivery.

The main symptom of placenta previa is vaginal bleeding that starts in the second or third trimester. There is often no associated pain.

It may be easy to miss this or attribute it to another issue. Any time vaginal bleeding occurs during pregnancy, it is important to receive medical attention.

When a doctor identifies placenta previa early in pregnancy, it can help them have a better understanding of the risks involved and whether a cesarean will be necessary.

A healthcare provider can spot placenta previa during a routine transabdominal ultrasound in the first or second trimester.

If a woman believes that she has placenta previa, due to vaginal bleeding. she should undergo an ultrasound, then possibly a digital examination.

An ultrasound of the abdomen can show a doctor whether it will be safe to perform an ultrasound of the vagina, called a transvaginal ultrasound.

During a transvaginal ultrasound, the healthcare provider or technician places the ultrasound tool within the vagina. This can give a better idea of where the placenta is positioned.

Experts have identified transvaginal ultrasound as the best method of identifying the type of placenta previa.

Women at greater risk of placenta previa include those who:

  • are over 35 years old
  • are carrying multiple babies
  • have given birth before
  • have had uterine surgery before, such as a cesarean delivery or the removal of uterine fibroids
  • have what doctors call abnormal placental development
  • smoke tobacco
  • use cocaine

Placenta previa can be dangerous for the woman and the developing baby. Experts have found, for example, that early delivery early after placenta previa may increase the risk of damage or death for both.

However, the position of the placenta often changes, resolving the issue before delivery.

If placenta previa does not resolve, it can increase the risk of:

  • complications to fetal growth
  • bleeding — during the pregnancy, the delivery, or the period that immediately follows
  • premature delivery
  • placenta accreta, which involves the placenta growing into the wall of the uterus
  • damage to nearby organs
  • the need for the removal of the uterus, in 0.2% of placenta previa cases
  • mortality

People sometimes confuse placenta previa with placental abruption, both of which can cause bleeding after the first trimester.

Placental abruption involves the placenta separating from the uterine lining early, before delivery.

Other differences between the issues are:

Placenta previaPlacental abruption
Signsare not so obvious from the startdevelop quickly, with high intensity
Bleedingis visibleis both internal and visible
Symptoms, such as shock or anemiaare proportionate to visible blood lossare intense, compared with visible blood loss
Painis often not presentis present

The placenta may realign, resolving the previa, within 32–35 weeks of pregnancy, when the lower part of the uterus thins and stretches out.

If placenta previa does not resolve on its own, a cesarean delivery becomes necessary.

The operation is usually booked for 36–37 weeks of pregnancy. If there are complications, cesarean delivery may become necessary earlier.

A woman can reduce any vaginal bleeding by resting in bed and avoiding triggers, including strenuous exercise and sexual intercourse.

Some types of vaginal examination can induce or worsen bleeding in a woman with placenta previa, so the entire healthcare team should be aware of the issue.

There are no ways to prevent placenta previa.

If placenta previa causes heavy bleeding, the fetus may not be receiving enough blood, which can lead to further complications.

In pregnancies with placenta previa, there is a higher chance of premature delivery and low birth weight.

In addition, placenta previa is the most common cause of postpartum hemorrhage, which is heavy vaginal bleeding from a ruptured blood vessel that follows delivery. In some women, this is life threatening.

Placenta previa involves the placenta covering the opening of the uterus into the cervix. This obstruction may be partial or complete.

The placenta’s position can determine whether a woman will need to undergo cesarean delivery.

The main risk of placenta previa involves blood loss. It can lead to various complications, some of which can be life threatening.

If any vaginal bleeding occurs during pregnancy, it is important to consult a healthcare provider.