Placenta previa is a pregnancy complication where the placenta attaches low in the uterus and partially or completely covers the cervix, which is the opening to the birth canal. It affects roughly 4 to 5 out of every 1,000 births and is the leading cause of significant bleeding in the third trimester. The good news: many cases detected early in pregnancy resolve on their own as the uterus grows.
How the Placenta Ends Up in the Wrong Spot
During a normal pregnancy, the placenta typically attaches to the upper or side wall of the uterus, well away from the cervix. In placenta previa, it implants low enough to reach or cover the cervical opening. Doctors distinguish between two situations based on ultrasound measurements: if the placenta’s edge is within 20 millimeters of the cervical opening or overlaps it, that’s placenta previa. If it sits more than 20 millimeters away but still lower than usual, it’s called a low-lying placenta.
The placenta can cover the cervix completely or only partially. This distinction matters because it affects both the likelihood of resolution and the severity of potential bleeding.
Why It Happens
There’s no single cause, but several factors increase the risk. Being over 35, having had multiple pregnancies, and smoking all raise the odds. A history of cesarean sections is one of the strongest risk factors because the scarring on the uterine wall can influence where a future placenta implants. Other contributors include prior uterine procedures (such as a D&C), cocaine use, fertility treatments like IVF, and having had placenta previa in a previous pregnancy.
The Hallmark Symptom: Painless Bleeding
The classic sign is painless vaginal bleeding during the second or third trimester. This is what sets placenta previa apart from many other causes of pregnancy bleeding, which tend to involve cramping or abdominal pain. The bleeding can range from light spotting to heavy, and it may be triggered by intercourse, a vaginal exam, or the onset of labor. Sometimes there’s no identifiable trigger at all.
Not everyone with placenta previa bleeds. Some people have no symptoms and only learn about it during a routine ultrasound. But any vaginal bleeding after the first trimester warrants an ultrasound before any internal exam, because examining the cervix manually when a placenta is covering it can provoke serious hemorrhage.
How It’s Diagnosed
Placenta previa is diagnosed with ultrasound, most often during the routine anatomy scan around 18 to 20 weeks. A transabdominal ultrasound (the standard belly scan) usually identifies it, but a transvaginal ultrasound, where the probe is placed near the cervix, gives a more precise measurement of how close the placenta sits to the cervical opening.
Here’s something important: being told you have placenta previa at your 20-week scan does not necessarily mean you’ll still have it at delivery. At that gestational age, about 11 percent of patients show a placenta near or over the cervix. The vast majority of those cases resolve.
Most Cases Resolve Before Delivery
As the uterus expands during pregnancy, the lower segment stretches and the placenta effectively “migrates” upward, away from the cervix. This isn’t the placenta physically moving. It’s the uterine wall beneath it growing and pulling it higher. In a study of 366 cases diagnosed around 18 to 19 weeks, 84 percent of complete previas and 98 percent of marginal (partial) previas had resolved by an average of about 29 weeks.
Your provider will schedule follow-up ultrasounds, typically in the third trimester, to check whether the placenta has moved. If it has cleared the cervix by a comfortable margin, your pregnancy can proceed without the restrictions that come with an active previa diagnosis.
Risks for Mother and Baby
When placenta previa persists into the third trimester, it carries real risks. The most immediate concern is hemorrhage. Because the placenta sits over or near the cervix, any cervical changes, whether from contractions, dilation, or even routine stretching, can tear placental blood vessels. This bleeding can be sudden, heavy, and life-threatening.
Placenta previa also increases the chance of a related condition called placenta accreta spectrum, where the placenta grows too deeply into the uterine wall. This makes it difficult to separate during delivery, raising the risk of massive blood loss and, in severe cases, the need for a hysterectomy.
For the baby, the primary risks are preterm birth and restricted growth. About 5 percent of all preterm deliveries are linked to placenta previa, and episodes of bleeding before delivery are a strong predictor of early delivery. Babies born prematurely face the usual challenges of underdeveloped lungs, feeding difficulties, and time in the neonatal intensive care unit.
What Daily Life Looks Like With Placenta Previa
If your placenta previa hasn’t resolved and you’re not actively bleeding, your provider will likely put you on what’s called pelvic rest. That means no sexual intercourse, no tampons, and no douching. You’ll also be told to cut back on strenuous activities: no running, heavy lifting, squatting, or jumping. Some people are put on modified bed rest at home, depending on how close the placenta is to the cervix and whether there have been any bleeding episodes.
Expect more frequent prenatal appointments and ultrasounds than a typical pregnancy. Your care team will monitor the placenta’s position, your baby’s growth, and any signs of bleeding. If you do experience bleeding, even a small amount, contact your provider immediately. Heavy or uncontrollable bleeding may require hospitalization for monitoring or, if the situation is urgent, an early delivery.
How Delivery Works
A placenta that still covers or sits very close to the cervix at delivery makes vaginal birth unsafe, because the baby would need to pass through the placenta first, causing catastrophic bleeding. A cesarean section is the standard delivery method for persistent placenta previa. For stable patients with no active bleeding or other complications, the C-section is typically scheduled between 36 and 37 weeks to balance the baby’s maturity against the risk of an unplanned bleeding episode.
If significant bleeding occurs before the scheduled date, an emergency cesarean may be necessary regardless of gestational age. In cases where placenta accreta spectrum is suspected, the surgical team will prepare for the possibility of heavier-than-normal blood loss and potential hysterectomy. This sounds alarming, but knowing the diagnosis in advance allows the medical team to plan for these scenarios rather than being caught off guard.