• Placenta previa is a pregnancy complication in which the placenta is partially or completely implanted over the opening of the uterus.
• It can cause bleeding and difficulties during birth and may require a cesarean section if still present by the time of delivery – though it often resolves itself naturally as pregnancy advances.
• Placenta previa is typically detected in the second trimester during a routine prenatal ultrasound exam.
• Warning signs that you may have placenta previa include vaginal bleeding during the second and third trimester. But not all women diagnosed with placenta previa will experience bleeding.
• This article was reviewed by Mia Di Julio, MD, OB-GYN at Providence Saint John’s Health Center.
• Visit Insider’s homepage for more stories.

When you’re pregnant, any amount of vaginal bleeding is cause for concern. If the bleeding occurs during the second trimester, your doctor will perform an ultrasound to see if you have a condition called placenta previa.

Placenta previa is when your placenta is partially or totally covering the cervix. Here’s what causes it and who is most at risk.

What causes placenta previa

The placenta develops inside your uterus and serves as an essential life-support system during pregnancy. It allows the mother to transfer oxygen and nutrients to the fetus, as well as transfer waste products from the fetus back to the mother for removal.

In most pregnancies, the placenta will attach to the side of your uterus and move with the growing uterine muscle toward the top of the womb as the pregnancy progresses, which allows the cervix to open for delivery.

But in cases of placenta previa, Michael Dent, MD, an OB-GYN and CEO at HealthLynked, says the placenta implants low, about two centimeters from the cervical opening, which, in some cases, can cause complications such as bleeding throughout pregnancy and difficulties during birth.

The good news? Getting to this point is relatively rare. In fact, placenta previa only occurs in about 0.5% of pregnancies, says Dent. "Many women diagnosed with the condition find it resolves itself naturally as the pregnancy advances," he says.

But if it does not resolve on its own, the placenta will remain partially, or completely, over the cervical opening when you go into labor, and you will likely require a C-section.

A routine ultrasound will show placenta previa

Most cases of placenta previa are diagnosed during a routine prenatal ultrasound exam around 20 weeks, says Fady Khoury Collado, MD, an OB-GYN and gynecologic oncologist at NewYork-Presbyterian/Columbia University Irving Medical Center.

Some women will experience bleeding before this ultrasound, which often prompts them to seek medical attention. But many others will have no symptoms, and find out they have placenta previa during this routine exam.

Who's at risk of placenta previa

Pinpointing the cause of placenta previa is difficult. In fact, Collado says the exact reason for why some patients develop placenta previa remains unknown.

However, experts do see a higher rate of placenta previa in the following situations:

• The mother-to-be is 35 or older.
• This isn't the mother's first pregnancy.
• The mother has given birth via C-section in the past.
• The mother is carrying more than one fetus.
• The mother smoked regularly throughout her pregnancy.
• For more risk factors, visit Stanford Children's Health.

How to treat placenta previa

While there is no way to "fix" placenta previa prior to giving birth, your doctor will recommend ways to reduce the risk of bleeding for the remainder of your pregnancy.

The recommendations can vary from patient to patient, but Collado says they may include instructions to avoid strenuous activity, heavy lifting, and sexual intercourse.

If the bleeding is severe, your doctor may admit you to the hospital for bed rest and observation. While rare, some women will need to have an early delivery via C-section if heavy bleeding persists.

Related stories about pregnancy:

• A C-section is often quicker than a natural birth, but doctors only recommend it under certain circumstances
• How long it takes to recover from a C-section before you can drive a car, take a bath, have sex, and more
• Yes, pregnancy can cause acne. Here's when you will breakout the worst and how to treat it.
• When pregnancy cravings start and how long they last
• Yes, Tylenol is safe for pregnancy. Here's the right dosage
• Pregnancy symptoms you can expect each trimester of your pregnancy and how to alleviate them