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Complications & miscarriage

What is Placenta Previa? Everything You Need to Know

If you’re pregnant, you might have heard of placenta previa as a possible complication of pregnancy. But what causes this complication—and how serious is it?

In this post, we’ll review:

  • Signs and symptoms of placenta previa
  • Risk factors for placenta previa
  • When to call your doctor


Key takeaways

  • Placenta previa occurs when the placenta is abnormally positioned, covering the cervix either partially, or in full.
  • The most characteristic symptom is a painless, bright red bleeding.
  • Common risk factors are previous C-section delivery and age (under 20 or over 35).
  • In most cases, placenta previa can be managed and usually results in a healthy delivery.


What is the placenta?


Before we dive into the details of placenta previa, let’s step back to marvel at the wonder that is the placenta. It’s quite an incredible feat: your body develops an entirely new organ (and temporary organ) specifically to support your developing fetus! Though its full functionality is complex, the placenta essentially serves as a two-way filter between mother and baby, connecting fetus and uterine wall via the umbilical cord.

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As blood flows through your uterus, the placenta soaks up nutrients, oxygen, and immune molecules from your blood, passing these to the baby. And when wastes and carbon dioxide build up in the baby’s blood, these are passed back to your blood to be filtered out.

The placenta forms during implantation. More specifically, when a sperm fertilizes an egg, a structure called a blastocyst is created. When that blastocyst burrows into your uterine wall, the inner part of that structure becomes the embryo, and the outer portion becomes the placenta.


What is placenta previa?


Normally, the placenta attaches itself on the uterine wall in a way that does not obstruct the cervix. But sometimes, it places itself near or over the cervical opening. This can lead to a variety of issues such as perilously low blood pressure, bleeding, and premature labor.

The low position of the placenta can also cause it to separate from the uterine wall. And when that happens, the placenta can no longer support the baby before birth. So in this case, your doctor may recommend that you give birth via C-section.

How common is placenta previa?


The good news is that this condition is relatively rare, affecting 1 in every 200 pregnant women. And if you’re told that you have placenta previa early in your pregnancy, it’s usually no cause for concern as most of these early cases resolve on their own without any intervention.


What are the different types of placenta previa?


There are three types of placenta previa:


  • Complete previa occurs when the placenta completely covers the opening to the cervix.
  • Partial previa occurs when the placenta only partially covers the opening to the cervix.
  • Marginal placenta previa occurs when the placenta lies close to the cervical opening but does not cover it.


While your doctor may suggest a C-section for placenta previa, especially for complete previas, vaginal birth may be possible with partial and marginal previas, as shown by this study. (The study also found that the further away the placenta was from the cervix, the better the odds of a successful vaginal birth.)


What causes placenta previa?


While the exact causes of placenta previa are unknown, it’s believed that it could be from scarring on the uterus or the endometrium. This type of scarring can result from previous C-section deliveries or uterine fibroid removal.


What are the symptoms of placenta previa?


The most recognizable symptom of placenta previa is a sudden bright red bleeding that appears later in the pregnancy, most commonly around 32 weeks.

Other symptoms you may notice include:

  • premature contractions
  • the uterus is unusually large for gestational age


The good news is that in many cases, placenta previa can resolve itself, especially if it’s diagnosed early. As your uterus grows, it may push the placenta away from the cervical opening, moving it aside like a balloon.

But if the placenta is fully covering the cervical opening—or if the diagnosis is made later in the pregnancy—the less likely the condition is to resolve.


Are there risk factors for placenta previa?


Several risk factors may make you more susceptible to developing placenta previa during your pregnancy.

These risk factors include:

  • Placenta previa in a previous pregnancy.
  • Previous C-section or other surgical interventions to the belly or cervix.
  • Age: younger than 20 or older than 35 years old.
  • Multiples or babies that are positioned in a breech or transverse position.


Research has found some racial and ethnic differences in the prevalence of placenta previa. This study found that placenta previa was most common in Asian women (followed by Native American, Caucasian, and then Hispanic).


What are the concerns?


Usually, the placenta is delivered after the baby, when it separates from the wall of your uterus. This is why the placenta is commonly referred to as the “afterbirth.” But, if the placenta is covering or potentially blocking the cervical opening, there’s a risk that the placenta can tear, bleed, and causing the mother to hemorrhage, which can be life-threatening.

If your placenta is damaged or tears, you might experience bright red bleeding without pain. If that’s the case, your doctor will do an ultrasound to determine the cause of the bleeding.


How is placenta previa diagnosed?


Placenta previa is best diagnosed by ultrasound. Transvaginal ultrasound is usually recommended as it provides a much more accurate reading than a scan done through your abdomen. The ultrasound specialist will determine the actual distance from the placental edge to the what’s called the “internal cervical os,” which is the opening on the innermost side of the canal near the baby. If that distance seems too small, a follow-up examination in the third trimester may be recommended.


How is placenta previa treated?


Before labor, placenta previa is usually treated with bed rest. But sometimes, if the bleeding is excessive, you may need to go to the hospital, where blood transfusions are readily available, and issues like anemia can be treated quickly.

Your doctor will weigh a variety of factors when determining how to proceed: your age and health, the length of your pregnancy, the baby’s health, and whether or not the placenta has started to detach from the uterine wall. The goal of your treatment will be to reduce bleeding and the risk of preterm birth. Some doctors may recommend bed rest, although there isn’t much evidence that it works.

While some women with partial or marginal placenta previa can give birth vaginally, C-section is usually the best option for mothers-to-be with a complete placenta previa.


How do I cope with a diagnosis of placenta previa?


Even though a diagnosis of placenta previa can feel terrifying, rest assured that this condition can be managed well—and sometimes, it goes away on its own. The majority of women with this condition go on to have a smooth delivery, whether it’s a C-section or vaginal birth.

But if you experience any bleeding, pain, or contractions (that begin before 2 weeks prior to your due date), contact your doctor right away.


Aarthi Gobinath, PhD

Aarthi Gobinath earned her PhD in neuroscience from the University of British Columbia. Her research covers the ways that stress affects the male and female brain differently.

She tackled the issue of sex bias in research by looking at why standard treatments for depression don't always work in the case of postpartum depression. Her work has been covered by Vice and Massive Science.

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