What Is Placenta Previa?
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Medically reviewed by Renita White, MD
Placenta previa is when the placenta stays low in the uterus during pregnancy and blocks the cervix partially or completely. This potentially serious condition can interfere with delivering a baby vaginally.
Placenta previa occurs in about 1 in 200 pregnancies during the third trimester or late in the second trimester. It usually resolves on its own as the uterus grows, creating distance between the cervix and the placenta. However, in some cases, it can lead to life-threatening bleeding and premature labor.
Ongoing monitoring and proactive treatment help reduce the risk of possible complications.
Types of Placenta Previa
The placenta is a temporary organ that develops during pregnancy to provide vital nutrients, hormones, and oxygen to the fetus. During the first trimester of pregnancy, the placenta lies low in the uterus.
As the pregnancy progresses, it typically moves up higher in the abdomen, away from the cervical opening. This allows the cervix—the lower part of your uterus that connects your uterus and vagina—to dilate (open up) without being blocked during delivery. However, in some pregnancies, the placenta remains low.
A low-lying placenta is when the placenta is less than two centimeters (cm) from the cervix. About 90% of low-lying placentas resolve by the third trimester.
Placenta previa is usually diagnosed when the placenta either partly or completely covers the cervix. Though the terms "low-lying placenta" and "marginal placenta" are sometimes used interchangeably, the types are generally understood as follows:
Marginal placenta previa: The placenta reaches the cervix but does not cover any part of it.
Partial placenta previa: The placenta covers part of the cervix.
Complete placenta previa: The placenta covers the entire cervix.
Placenta Previa Symptoms
Vaginal bleeding is the most common symptom of placenta previa. This bleeding is typically bright red and usually occurs during the third trimester. It may be painless or accompanied by cramping. In many cases, the bleeding stops and starts.
Other symptoms of placenta previa may include:
A fetus in breech position (i.e., bottom or feet down rather than head down)
Premature uterine contractions
Causes and Risk Factors
The cause of placenta previa is unknown. Any pregnant person can develop it at some point during pregnancy. However, certain factors increase your risk of developing this condition.
Risk factors include:
Conception via in vitro fertilization (IVF)
Previously giving birth to multiples (for example, twins or triplets)
Multiple births
Previous uterine surgery, such as a Cesarean section (C-section)
Maternal age greater than 35 years old
Smoking
Using cocaine
Related:Health Risks Associated With Having a Baby at 40 and Older
Diagnosis
Placenta previa is often diagnosed through routine pregnancy sonograms in the first and second trimesters. Transvaginal sonograms are performed internally, through the vagina. This type of sonogram is generally considered more accurate than transabdominal sonograms, which are performed externally through the abdomen.
Your healthcare provider will also likely check for placenta accreta spectrum (PAS), as this condition is associated with placenta previa. PAS is when the placenta attaches too deeply to the uterine wall. This is a serious condition that can lead to hemorrhaging (heavy bleeding). It has more delivery risks than placenta previa.
Placenta Previa Treatment
If you have a low-lying placenta at your 20-week fetal anatomy scan, your healthcare provider will most likely ask you to come in for follow-up ultrasounds, usually around 28 weeks and 32 weeks of pregnancy, to make sure the placenta has shifted into the correct position.
Treatment for placenta previa focuses on preventing bleeding and premature labor, which is labor that occurs before 37 weeks of pregnancy. Your healthcare provider might recommend the following:
Avoid exercise, heavy lifting, and standing for long periods of time (more than four hours straight)
Focus on pelvic rest, including avoiding sexual intercourse and pelvic exams
Limit travel
If placenta previa is still present at 36 weeks, your healthcare provider will likely schedule a C-section, which is surgery to deliver the baby. Delivering vaginally when the placenta covers the cervix can cause severe hemorrhaging. This can require a blood transfusion, particularly with complete or partial placenta previa. A blood transfusion is when you receive healthy blood through an intravenous (IV) line.
Depending on the severity of your symptoms, you might have a C-section before 36 weeks. You also may also need to stay in the hospital longer—either before or after giving birth—to be monitored for bleeding and other possible complications.
People with placenta previa have a higher risk of premature labor, which can lead to complications for newborns. Your healthcare provider may administer steroid shots to help your baby’s lungs develop faster. They may also give you medication to prevent early labor.
Prevention
There’s no way to prevent placenta previa. However, you can lower your risk by avoiding smoking and recreational drug use during pregnancy.
Getting prenatal care can also help identify the problem early so you can take steps to avoid potential complications. Talk to your healthcare provider about potential risk factors for placenta previa, such as past surgeries and multiple births.
Living With Placenta Previa
Most cases of a low-lying placenta or placenta previa resolve on their own as your pregnancy continues. Your healthcare provider will monitor the position of your placenta to make sure it's not blocking your cervix in preparation for labor and delivery.
If your placenta remains low, you could experience severe bleeding either before, during, or just after labor. You also might be more likely to have a preterm delivery, which could lead to potential complications for your newborn.
Placenta previa can also increase your risk of developing placenta accreta, a condition that can lead to potentially life-threatening bleeding during or after childbirth.
If you have a confirmed or suspected diagnosis of placenta previa, it’s important to follow your healthcare provider’s recommendations in terms of pelvic rest and method of delivery.
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