A low placenta means the placenta has attached in the lower part of your uterus, close to or covering the opening of your cervix. This is common at the 20-week anatomy scan, and roughly 90% of low-lying placentas move upward on their own by the third trimester as the uterus grows. If it stays low, it can affect how you deliver and raises the risk of bleeding later in pregnancy.
Low-Lying Placenta vs. Placenta Previa
These two terms describe different degrees of the same issue. A low-lying placenta sits within 20 millimeters of the cervical opening but doesn’t cover it. Placenta previa means the placenta partially or completely covers the cervical opening. The distinction matters because it changes what your care team recommends for delivery and how closely they monitor you.
At your mid-pregnancy ultrasound, the technician measures the distance between the edge of the placenta and the internal opening of the cervix. If that distance is greater than 20 mm, the placenta is considered normally positioned. Between 1 and 20 mm, it’s low-lying. At zero or overlapping, it’s previa.
Why Most Low Placentas Resolve on Their Own
The placenta doesn’t literally slide up the uterine wall. What happens is that the lower segment of the uterus stretches and lengthens as pregnancy progresses, effectively pulling the attachment site upward and away from the cervix. This process, sometimes called placental migration, resolves the issue in about 9 out of 10 cases diagnosed at the 20-week scan. A follow-up ultrasound, typically around 32 to 36 weeks, confirms whether the placenta has moved far enough from the cervix.
If your placenta was only slightly low at 20 weeks, the odds of it clearing the cervix by the third trimester are very good. Placentas that completely cover the cervix are less likely to resolve, but many still do.
How It’s Diagnosed and Monitored
Transvaginal ultrasound is the most accurate way to measure the distance between the placenta and the cervix. A standard abdominal ultrasound correctly identifies a low placenta only about 27% of the time at mid-pregnancy, so if there’s any question about placental position, a transvaginal scan gives a much clearer picture. Despite the name, this type of ultrasound is safe and doesn’t disturb the placenta or cervix.
You’ll typically have at least one follow-up scan in the third trimester to check whether the placenta has migrated. If it hasn’t moved enough, your provider may schedule additional imaging closer to your due date to finalize delivery plans.
Symptoms to Be Aware Of
The hallmark symptom is bright red vaginal bleeding, usually painless, after 20 weeks. Sometimes it starts as light spotting before a larger episode. Bleeding can be triggered by sex, a cervical exam, or contractions, but often there’s no obvious trigger at all. Some women with a low placenta never bleed during pregnancy, and for others, bleeding doesn’t start until labor begins.
If you experience any vaginal bleeding in the second or third trimester, contact your provider right away. Painless bleeding in particular is a classic sign that the placenta’s position is involved.
Risk Factors
Certain factors make a low placenta more likely. The strongest is a history of cesarean section: women who’ve had a prior C-section have about six times the odds of developing placenta previa compared to those who haven’t. This is because the scar tissue on the uterine wall can influence where a future placenta implants.
Other risk factors include:
- Maternal age over 30: nearly four times the odds compared to younger women
- Previous uterine procedures such as dilation and curettage (D&C)
- Fertility treatments like IVF
- Multiple pregnancies (twins, triplets)
- Smoking
- History of placenta previa in a previous pregnancy
Having one or more of these factors doesn’t mean you’ll definitely have a problem. It simply means your care team may pay closer attention to placental position throughout your pregnancy.
What It Means for Delivery
Whether you can deliver vaginally depends on how close the placenta remains to the cervix in late pregnancy. A systematic review of the research found that when the placenta sits 11 to 20 mm from the cervical opening, vaginal delivery is successful about 85% of the time. Even at a distance of 0 to 10 mm, vaginal delivery was still achieved in 43% of cases, though a planned cesarean is more commonly recommended at that range.
Once the placenta is more than 20 mm from the cervix, it’s no longer considered low-lying, and delivery planning proceeds as normal. If the placenta still covers or sits very close to the cervix near your due date, a cesarean is the safest option because labor contractions could cause the placenta to detach and bleed heavily.
Possible Complications
When a low placenta or placenta previa persists, the primary concern is bleeding. As the lower uterus stretches and the cervix begins to thin in preparation for delivery, the placenta can partially separate from the uterine wall, causing hemorrhage. This bleeding can happen before labor, during labor, or after delivery, and in severe cases may require a blood transfusion.
For the baby, a persistent previa significantly raises the risk of preterm birth. In one study, nearly half of women with placenta previa delivered before 37 weeks. Babies born to mothers with previa also face higher rates of restricted growth in the womb and breathing difficulties after birth, largely because of prematurity.
Activity Changes You May Be Asked to Make
If your placenta is low and you’ve had bleeding, your provider will likely recommend pelvic rest. This means avoiding sex, not placing anything in the vagina, and skipping strenuous exercise. Depending on your situation, you may also be asked to limit how long you stand or walk at a stretch (often capped at about 30 minutes), avoid lifting anything over 20 pounds, reduce stair climbing, and skip long-distance air travel.
These restrictions vary widely from person to person. Some women with a low-lying placenta and no bleeding are told to continue normal activities while waiting for the follow-up scan. Others with active bleeding or a complete previa may need more significant modifications. The specifics depend on how low the placenta is, whether you’ve bled, and how far along you are, so it’s worth asking your provider exactly which activities to avoid rather than guessing.
What to Expect Going Forward
If you’ve just learned your placenta is low at your anatomy scan, the most likely outcome is that it resolves without any intervention. Your provider will schedule a follow-up ultrasound in the third trimester, and in the meantime, you’ll watch for any vaginal bleeding. For the small percentage of cases where the placenta stays low, the focus shifts to careful delivery planning and monitoring for bleeding. With proper monitoring, the vast majority of women with a low placenta or even full placenta previa have safe deliveries and healthy babies.