Placenta previa is a pregnancy complication where the placenta attaches low in the uterus and partially or completely covers the cervix, the opening the baby needs to pass through during delivery. It affects roughly 1 in 200 pregnancies at the time of delivery, though it’s detected on ultrasound far more often earlier in pregnancy, with reported rates of 1 to 15% during second-trimester scans. The good news: the vast majority of cases diagnosed in the second trimester resolve on their own before the baby arrives.
How Placenta Previa Is Diagnosed
Most cases are caught during a routine anatomy ultrasound around 18 to 20 weeks. The key measurement is the distance between the edge of the placenta and the internal opening of the cervix. If the placenta sits within 20 millimeters of that opening, it’s considered low-lying. If it reaches or covers the opening entirely, it’s classified as placenta previa.
Transvaginal ultrasound, where a probe is placed in the vagina, gives the most accurate picture. Despite what you might expect, this type of ultrasound is safe with placenta previa and actually provides better detail about the placenta’s position and how much of the cervix it covers.
When previa is found at the mid-pregnancy scan, follow-up ultrasounds are typically scheduled around 28 to 30 weeks and again at 32 to 34 weeks to check whether the placenta has moved away from the cervix.
Why Most Early Diagnoses Resolve
As the uterus grows during pregnancy, the lower portion stretches and expands. This growth effectively pulls the placenta upward and away from the cervix, even though the placenta itself isn’t actually moving. A large study found that more than 80% of complete placenta previas diagnosed in the second trimester resolved before delivery. For marginal previas, where only the edge of the placenta reaches the cervix, the resolution rate was 98%.
Among those that did resolve, the average gestational age at resolution was about 28 weeks, and 90% had resolved by around 34 weeks. So if your provider flags a low placenta at your 20-week scan, the odds are strongly in favor of it correcting itself. You’ll simply be monitored with additional ultrasounds to confirm.
The Main Symptom: Painless Bleeding
The hallmark of placenta previa is painless vaginal bleeding during the second or third trimester. The blood is typically bright red and can range from light spotting to heavy flow. Unlike many other causes of pregnancy bleeding, there are usually no cramps or contractions accompanying it.
Bleeding episodes can be triggered by intercourse, a vaginal exam, or the onset of labor contractions, but they can also happen with no identifiable cause. Some women with confirmed previa never experience any bleeding at all, while others have repeated episodes. Any vaginal bleeding after 20 weeks warrants prompt medical evaluation, regardless of whether it’s painful.
Who Is at Higher Risk
Several factors significantly raise the likelihood of developing placenta previa. The strongest predictor is having had placenta previa in a previous pregnancy, which increases the odds 11-fold. A history of cesarean delivery increases risk about sixfold, likely because the uterine scar changes how the placenta implants. Previous uterine procedures like dilation and curettage carry a fivefold increase in odds.
Maternal age over 30 roughly quadruples the risk compared to younger women. Conceiving through assisted reproductive technologies such as IVF is associated with nearly four times higher odds as well. Carrying twins or multiples may raise risk slightly, though this association is less statistically clear.
Why Placenta Previa Matters
The primary concern is hemorrhage. Because the placenta sits over or near the cervix, any thinning or opening of the cervix can disrupt placental blood vessels, causing bleeding that can become severe quickly. This risk exists throughout the second and third trimesters and is especially high during labor, which is why vaginal delivery is not an option when previa persists.
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 is more common when previa occurs alongside a uterine scar from a prior cesarean. Accreta makes the placenta difficult or impossible to detach after delivery and can lead to life-threatening bleeding that sometimes requires removal of the uterus. Ultrasound monitoring throughout the third trimester helps identify signs of accreta so the delivery team can plan accordingly.
For the baby, the main risks are indirect: preterm birth if heavy bleeding forces an early delivery, and the complications that come with prematurity. If bleeding episodes occur before the baby’s lungs are fully mature, steroid injections may be given to accelerate lung development.
What Daily Life Looks Like With Previa
If placenta previa is diagnosed and persists into the third trimester, your provider will likely recommend activity restrictions. “Pelvic rest” is the most common instruction, meaning nothing placed in the vagina and often no sexual intercourse or orgasm. Beyond that, you may be advised to stop heavy lifting and strenuous exercise. Some women are put on modified bed rest, particularly after a bleeding episode, though the evidence for strict bed rest improving outcomes is limited.
The specifics vary depending on whether you’ve had any bleeding and how much of the cervix the placenta covers. Women who have had significant bleeding episodes may be hospitalized for closer monitoring, especially as they approach the third trimester, so that emergency delivery is possible if heavy bleeding occurs.
How Delivery Works
When placenta previa persists to the end of pregnancy, a cesarean delivery is necessary. Attempting vaginal delivery would mean the baby passes through or past the placenta, risking catastrophic bleeding for both mother and baby.
For uncomplicated, stable previa, delivery is typically scheduled between 36 and 37 weeks. This timing balances the baby’s maturity against the rising risk of spontaneous bleeding or labor as the due date approaches. If there’s suspicion of placenta accreta based on ultrasound findings, a surgical team experienced in managing major bleeding will be assembled, and the delivery may be planned at a hospital with advanced blood bank capabilities.
For women whose previa resolves before the third trimester and whose placenta has moved well clear of the cervix, vaginal delivery remains a possibility, and their pregnancy management returns to normal.