Placenta previa is when the placenta either partially or completely covers the cervix. It is usually detected during an ultrasound in the second or third trimester of pregnancy, or when a pregnant person presents with symptoms of bright-red vaginal bleeding that is typically painless.

It is normal for the placenta to be lower in the uterus at the start of pregnancy and move further away from the cervix as the pregnancy progresses and the uterus grows and stretches. This can allow for a vaginal birth. In some cases, however, the placenta remains covering the cervix, obstructing the entrance to the vaginal canal.

Placenta previa is managed by an OBGYN or other obstetric provider. In cases that are not complicated by vaginal bleeding, management may include rest and avoiding sexual intercourse. Total bedrest, however, has not been shown to be beneficial in people with placenta previa ^[1]. In the case of heavy bleeding, it may be necessary to be admitted to the hospital for evaluation and monitoring. In the most severe cases, an emergency cesarean delivery (c-section) may be indicated.

Common symptoms

The main symptoms of placenta previa are:

Bright-red vaginal bleeding that is usually painless;
Sudden onset of bleeding that can be light or heavy;
Bleeding after sex;
Bleeding with contractions or with sharp belly pain.

Symptoms of placenta previa are more common after 20 weeks of pregnancy, and may start as light bleeding or spotting prior to the onset of heavier bleeding.

Placenta previa can increase the risk for a condition called placenta accreta, in which the placenta adheres to the myometrium (the middle layer of the uterine wall) and can lead to severe hemorrhage.

It is important to seek urgent medical attention for any signs of placenta previa, as symptoms of placenta previa can be similar to those of placental abruption. An ultrasound will be done as part of the workup to confirm the location of the placenta.

Confirming the diagnosis

The diagnosis of placenta previa is made by an obstetrician based on symptoms, medical history, a pelvic exam, and an ultrasound. Diagnosis is confirmed by a transvaginal ultrasound, which is safe to perform and can provide more detailed images of the placenta to determine the type of placenta previa.

In pregnant people without symptoms, placenta previa can be diagnosed on a routine prenatal ultrasound during the first or second trimester of pregnancy. A diagnosis of placenta previa warrants a repeat ultrasound at 28 to 32 weeks to determine if the condition persists.

Types of placenta previa

Placenta previa can be classified according to its location in the uterus. The different types of placenta previa include:

Complete placenta previa: the placenta completely covers the cervix;
Partial placenta previa: the placenta partially covers the cervix;
Marginal placenta previa: the edge of placenta touches the cervical os (opening) but does not cover it;
Low-lying placenta: the edge of the placenta is close to the cervix (1-20 mm away) but does not touch it.

While there may be no symptoms, placenta previa can cause vaginal bleeding and increase the risk for premature birth and other complications during pregnancy and birth.

Common causes

The exact cause of placenta previa is not completely understood, however there appears to be an association with endometrial lesions (damage to the lining of the uterus) and uterine scarring, from trauma or previous surgery for example.

Risk factors for placenta previa include:

History of placenta previa in a prior pregnancy;
Maternal age over 35;
History of a prior c-section;
History of uterine surgery;
Past history of uterine curettage;
Twin pregnancy;
Tobacco use;
Use of illicit substances, such as cocaine.

Pregnancies conceived using assisted reproductive technology (such as IVF) have also been shown to have an increased risk of placenta previa.

Treatment options

Placenta previa is managed by an obstetrician, and may be managed in the hospital or at home depending on gestational age and amount of bleeding. Management of placenta previa may involve:

Avoiding strenuous activity and prolonged standing;
Being taken out of work for the remainder of the pregnancy;
Avoiding sexual activity.

A c-section is recommended between 36 and 37 weeks of pregnancy, in order to minimize the risk of bleeding. An emergency c-section may be necessary at an earlier gestation if the pregnant person or fetus is at risk.

Heavy vaginal bleeding requires admission to the hospital, where the pregnancy can be monitored and a blood transfusion or emergency c-section can be performed if necessary.

In cases with a high risk of preterm labor, a doctor can administer medications that can accelerate fetal lung development and may consider other medications to decrease the risk of preterm birth.

Possible risks

The biggest risk involved with placenta previa is the potential for heavy vaginal bleeding (hemorrhage). This can happen during pregnancy, labor and birth, or in the immediate postpartum period, and could be life threatening.

Placenta previa can increase the chances of placenta accreta, which is when the placenta adheres (is stuck) to the wall of the uterus, making it difficult for the placenta to come out following the birth. This can lead to a hemorrhage requiring blood transfusions or, in the most serious cases, total hysterectomy (removal of the uterus). Further complications include a longer hospital stay and an increased risk for infection.

Placenta previa is associated with an increased risk of preterm birth and neonatal complications such as low birth weight, lower APGAR scores, NICU admission, longer hospital stay, and, in some cases, the need for a blood transfusion.

Mode of delivery

A c-section is indicated in pregnancies complicated by placenta previa and is performed between 36 and 37 weeks gestation. This is due to the risk for hemorrhage during a vaginal delivery if the fetus attempts to pass through the cervix, which is covered by the placenta.

A vaginal birth is possible when the edge of the placenta is at least 2 cm from the cervix, and will depend on maternal health status and evaluation by an OBGYN.