When the placenta covers the opening of the cervix during pregnancy, the condition is called placenta previa. It affects roughly 3 to 5 out of every 1,000 pregnancies worldwide, and the rate has been climbing as cesarean delivery becomes more common. Placenta previa can involve a complete or partial covering of the cervical opening, and it matters because it can cause significant bleeding and changes how your baby is delivered.
How Placenta Previa Is Classified
The placenta normally attaches to the upper or side wall of the uterus, well away from the cervix. In placenta previa, it implants low enough to cover the internal opening of the cervix, which is the passage your baby would need to move through during labor. Doctors distinguish between a few variations based on how much of that opening is blocked.
Complete placenta previa means the placenta entirely covers the cervical opening. Partial previa means it covers only a portion. A marginal previa describes a placenta whose edge sits within 2 centimeters of the opening without actually covering it. And a low-lying placenta is one that falls within about 2 to 3.5 centimeters of the opening. These distinctions matter because they influence the likelihood of bleeding, whether the placenta will move on its own as the uterus grows, and whether a vaginal delivery is possible.
The Main Symptom: Painless Bleeding
The hallmark sign is bright red vaginal bleeding, typically without pain, occurring after 20 weeks of pregnancy. For many people, there is no obvious trigger. The bleeding can start as light spotting and then progress to a heavier episode later. In some cases, it is brought on by sex, a medical exam, or contractions that cause cramping. A smaller number of people experience no bleeding at all until labor begins.
What makes this bleeding distinctive is its painless quality. Most causes of vaginal bleeding later in pregnancy involve cramping or abdominal pain. With placenta previa, the bleeding often arrives without warning and without discomfort, which can be both confusing and alarming. Any bright red bleeding in the second half of pregnancy warrants prompt evaluation.
How It Is Diagnosed
Placenta previa is almost always found on ultrasound. Many cases are first spotted during the routine anatomy scan around 18 to 20 weeks. The sonographer measures the distance between the leading edge of the placenta and the internal cervical opening. If the placenta is within 20 millimeters of that opening, it is flagged as low-lying, and a follow-up ultrasound is recommended at 34 to 36 weeks to see whether the placenta has moved.
Transvaginal ultrasound (where the probe is placed inside the vagina rather than on the abdomen) gives a clearer view of the relationship between the placenta and cervix. Despite what some people worry about, this type of ultrasound is safe and does not increase the risk of bleeding. It is the preferred method when a low placenta is suspected.
Why the Placenta Sometimes Moves
A low-lying placenta found at 18 to 20 weeks does not necessarily mean you will still have placenta previa at delivery. As the uterus expands during the second and third trimesters, the lower part of the uterine wall stretches and thins. This stretching can pull the placenta upward and away from the cervix, a phenomenon often called placental “migration.” The placenta itself does not detach and physically relocate. Instead, the tissue it is anchored to shifts as the uterus grows.
The majority of low-lying placentas identified at the mid-pregnancy scan resolve by the third trimester. Complete previa is less likely to resolve than a marginal or low-lying placenta, but even some complete cases clear the cervix by 36 weeks. This is why doctors wait until late in the third trimester to make final delivery plans.
Risk Factors
The single strongest risk factor is a prior cesarean delivery. The scar left on the uterine wall appears to attract placental implantation in that lower area, and the risk increases with each additional cesarean. Other uterine surgeries that leave scars, such as fibroid removal, carry a similar effect.
Smoking during pregnancy roughly doubles the risk. Research adjusting for other variables like maternal age, number of previous pregnancies, and prior abortions found that the link between smoking and placenta previa held consistently, with the risk climbing slightly with heavier smoking. Cocaine use has also been associated with higher rates, though the evidence is less definitive. Older maternal age, carrying multiples, and having had several prior pregnancies all independently raise the likelihood as well.
Complications for Mother and Baby
The primary danger is hemorrhage. In a large study of women with placenta previa who had cesarean deliveries (and did not have the related condition placenta accreta), 19% experienced significant hemorrhagic complications. Compared to women without previa, those with the condition were nearly four times as likely to need a blood transfusion and about three times as likely to need medication to control uterine bleeding after delivery. About 2% required a hysterectomy, and 20% were delivered on an emergency basis because of antenatal bleeding.
Babies are affected too. Pregnancies with placenta previa tend to end earlier, with an average delivery around 35 weeks compared to nearly 38 weeks in uncomplicated pregnancies. That gap of roughly two and a half weeks translates to lower birth weights: an average of about 2,500 grams (5.5 pounds) versus nearly 3,000 grams (6.6 pounds). Premature birth brings its own set of challenges, including potential breathing difficulties and longer stays in the neonatal unit.
There is also an important link between placenta previa and a condition called placenta accreta, where the placenta grows too deeply into the uterine wall and cannot detach normally after delivery. The risk of accreta is highest when previa occurs in a uterus that already has cesarean scars. When both conditions are present, the likelihood of severe bleeding and hysterectomy rises substantially.
What to Expect During Management
If placenta previa is confirmed in the second trimester, your care team will likely recommend pelvic rest, which means avoiding sex, tampons, and anything placed in the vagina. Depending on how much bleeding you have had, you may also be advised to limit physical activity or, in more serious cases, go on modified bed rest. The goal is to minimize anything that could trigger or worsen bleeding and to keep the pregnancy going as long as safely possible.
If you have a significant bleeding episode, you may be admitted to the hospital for monitoring, even if the bleeding stops on its own. Some people with recurrent bleeds spend the final weeks of pregnancy in the hospital so that an emergency delivery can happen quickly if needed. Steroid injections are commonly given between 24 and 34 weeks to help the baby’s lungs mature in case early delivery becomes necessary.
How Delivery Works
When the placenta fully or partially covers the cervix and has not moved by the third trimester, a cesarean delivery is the only safe option. Attempting vaginal birth would mean the baby passes through the placenta first, causing life-threatening bleeding for both mother and child. The cesarean is typically scheduled between 36 and 37 weeks to balance the baby’s maturity against the risk of spontaneous labor and uncontrolled bleeding.
If the placenta has moved to a marginal or low-lying position but is no longer covering the cervical opening, a vaginal delivery may be considered, depending on the exact distance between the placental edge and the cervix. Your provider will make this call based on the final ultrasound measurements and whether you have had any bleeding episodes. In any scenario where previa persists, the surgical team prepares for the possibility of heavier-than-normal blood loss, often having blood products ready in advance.