Seeing the words “placenta previa” on an ultrasound report can be frightening, especially if you are in the second or third trimester and are wondering whether it means you will need a C-section.
The good news is that a placenta that looks low early in pregnancy may change position relative to the cervix as the uterus grows. However, true placenta previa that persists later in pregnancy is important because it can cause significant vaginal bleeding and may block the baby’s route through the birth canal.
Ultrasound is the main way doctors identify and monitor placenta previa. A transvaginal ultrasound may be used to accurately determine the relationship between the placenta and the cervix, and it is considered safe during pregnancy.
So, what exactly does placenta previa mean, and what happens next?
What Is Placenta Previa?
The placenta is an organ that develops during pregnancy and supplies your baby with oxygen and nutrients through the umbilical cord.
Normally, the placenta attaches to the upper or middle portion of the uterus. With placenta previa, the placenta lies very close to the cervix and, in the strictest definition, covers the internal cervical opening (internal os) either partially or completely.
Current RCOG guidance distinguishes placenta previa from a low-lying placenta based on the placental edge’s relationship to the internal cervical opening. A placenta is considered low-lying when its leading edge is less than 20 mm from the internal os, while placenta previa refers to placental tissue covering the internal os.
This distinction is important because the placenta’s exact position later in pregnancy helps determine whether vaginal delivery is possible.
Placenta Previa vs. Low-Lying Placenta
These two terms are sometimes used interchangeably in ultrasound conversations, but they are not exactly the same.
Low-Lying Placenta
The placenta is close to the cervix but does not cover the cervical opening.
Placenta Previa
The placenta covers the internal cervical opening.
For example, an ultrasound report may state:
“Placenta is low-lying, with its lower edge measuring 12 mm from the internal os.”
That is different from:
“Placenta previa with placenta covering the internal os.”
The measurement and wording matter because they help your obstetrician determine the appropriate follow-up and delivery plan.
How Is Placenta Previa Diagnosed on Ultrasound?
Placental location is routinely assessed during the mid-pregnancy anatomy ultrasound, generally around 18–22 weeks.
If the placenta appears low, the sonographer or physician may evaluate the lower uterine segment and measure the distance between the placental edge and the internal cervical os.
A transabdominal ultrasound can identify a low placenta, but when the exact relationship between the placenta and cervix needs clarification, transvaginal ultrasound provides a more accurate assessment. It is considered safe when performed appropriately.
Also Read : Placenta Is Low-Lying at 20 Weeks: What Happens Next?
This is important because an ultrasound finding at 20 weeks does not necessarily predict the final placental position at delivery.
Can Placenta Previa Resolve During Pregnancy?
Yes—especially when it is detected earlier in pregnancy.
Many placentas that appear low during the second trimester are no longer low later in pregnancy. RCOG patient information states that approximately 9 out of 10 women with a low-lying placenta at the 20-week ultrasound will no longer have a low-lying placenta at their follow-up scan.
This happens because the uterus continues to expand and the lower uterine segment develops as pregnancy progresses. The placenta itself does not literally crawl upward; instead, its position becomes farther from the cervix relative to the growing uterus.
However, persistent placenta previa later in pregnancy requires closer monitoring because it can create a significant bleeding risk.
What Happens If Placenta Previa Is Found at 20 Weeks?
If placenta previa or a low-lying placenta is identified during your 20-week ultrasound, your doctor will usually recommend follow-up imaging rather than immediately deciding on delivery.
The follow-up ultrasound checks whether:
- The placenta is still close to the cervix
- The placenta continues to cover the internal os
- The distance from the placental edge to the cervix has increased
- There are ultrasound features suggesting placenta accreta spectrum
- Additional pregnancy monitoring is needed
RCOG guidance recommends a follow-up scan around 32 weeks for a low-lying placenta identified at the 20-week scan, with another scan around 36 weeks if it remains low.
The exact schedule used in the United States may vary depending on your OB-GYN, maternal-fetal medicine specialist, symptoms, ultrasound findings and medical history.
Is Transvaginal Ultrasound Safe With Placenta Previa?
Many pregnant patients worry when their doctor recommends a transvaginal ultrasound after finding a low placenta.
A properly performed transvaginal ultrasound is considered safe and can provide a much clearer view of the cervix and lower edge of the placenta than an abdominal scan in some situations.
It is especially useful when the placental edge needs to be measured accurately.
If placenta previa is suspected in a patient with vaginal bleeding, the location of the placenta should be established before performing a digital cervical examination because manipulation can cause significant bleeding.
What Are the Symptoms of Placenta Previa?
The classic symptom of placenta previa is painless vaginal bleeding during the second half of pregnancy, particularly in the third trimester.
The bleeding can sometimes occur suddenly and may be heavy.
Some people with placenta previa have no symptoms at all and are diagnosed during a routine ultrasound.
Bleeding may sometimes occur after sexual intercourse because the cervix and lower uterine segment can be affected by the low placental location.
If you have a known low-lying placenta or placenta previa and experience vaginal bleeding, contractions or pelvic pain, contact your maternity care team immediately. Heavy bleeding is an emergency.
Why Is Placenta Previa a Concern?
The biggest concern is bleeding.
As pregnancy progresses, the lower portion of the uterus changes and the cervix begins preparing for labor. If the placenta is positioned over or very close to the cervix, this process can disrupt placental blood vessels and lead to potentially severe bleeding.
Placenta previa is associated with an increased risk of:
- Antepartum vaginal bleeding
- Emergency hospitalization
- Preterm birth
- Blood transfusion
- Cesarean delivery
- Postpartum hemorrhage
The baby may also need to be delivered early if significant bleeding occurs.
This is why persistent placenta previa requires careful prenatal planning.
Does Placenta Previa Mean I Need a C-Section?
If true placenta previa persists near delivery and the placenta covers the cervical opening, a planned cesarean delivery is generally recommended because the placenta can obstruct the baby’s passage through the birth canal and labor can trigger severe bleeding.
For stable patients with placenta previa without bleeding or other complications, SMFM guidance endorsed by ACOG recommends delivery at 36 0/7 to 37 6/7 weeks.
However, this does not mean that every low placenta found at 20 weeks automatically requires a C-section.
If the placenta later moves sufficiently away from the cervix, vaginal birth may be possible depending on the exact ultrasound findings and your overall clinical situation.
The final delivery decision should therefore be based on the placenta’s position later in pregnancy, not simply on a 20-week ultrasound finding.
What If I Have Bleeding With Placenta Previa?
If you have known placenta previa and develop vaginal bleeding, do not wait for your next appointment.
Contact your healthcare provider or go to your maternity hospital/emergency department as instructed.
Seek urgent medical attention for:
- Heavy vaginal bleeding
- Repeated bleeding
- Dizziness or fainting
- Severe abdominal or pelvic pain
- Regular contractions
- Leakage of fluid
- Decreased fetal movement later in pregnancy
Placenta previa bleeding can sometimes become severe very quickly, so medical evaluation is important.
Can You Have Sex or Exercise With Placenta Previa?
This is an area where individualized medical advice is important.
If your ultrasound shows a low placenta, ask your OB-GYN specifically about:
- Sexual intercourse
- Exercise
- Heavy lifting
- Travel
- Work activities
- Any restrictions you may need
There is no single activity restriction that applies to every patient with a low placenta.
If you have experienced bleeding or have persistent placenta previa, your healthcare provider may recommend avoiding certain activities because bleeding can be triggered by irritation of the cervix or lower uterine segment.
Do not assume that strict bed rest is necessary unless your medical team specifically recommends it.
Who Is More Likely to Develop Placenta Previa?
Several factors are associated with a higher likelihood of placenta previa.
These include:
- Previous C-section
- Multiple previous pregnancies
- Previous uterine procedures
- Smoking
- Increasing maternal age
- Certain fertility treatments
A previous cesarean delivery is particularly important because a placenta located over a uterine scar may also raise concern for placenta accreta spectrum (PAS).
Placenta accreta spectrum occurs when the placenta attaches abnormally deeply into the uterine wall. It can cause severe bleeding during delivery, so identifying patients at increased risk before delivery is important.
Also Read : Echogenic Focus in Fetal Heart: What Does It Mean?
Importantly, having a previous C-section does not mean you will automatically develop placenta accreta.
Placenta Previa and Placenta Accreta: Are They the Same?
No.
Placenta previa describes the placenta’s location in relation to the cervix.
Placenta accreta spectrum describes abnormal attachment or invasion of the placenta into the uterine wall.
They can occur together, particularly when placenta previa overlies a previous C-section scar, but they are different conditions.
When placenta accreta spectrum is suspected, specialized ultrasound assessment and delivery planning may be recommended. ACOG and SMFM emphasize that antenatal recognition and planned care at an appropriately equipped center can improve outcomes.
RCOG’s updated 2026 guidance also emphasizes earlier risk detection, expert ultrasound assessment and multidisciplinary planning for pregnancies at high risk of placenta accreta spectrum.
What Happens Near the End of Pregnancy?
If placenta previa persists into the third trimester, your healthcare team will carefully assess the placenta and cervix.
A transvaginal ultrasound may be used to determine the exact relationship between the placental edge and internal cervical os.
Your care team may also review:
- Any previous episodes of bleeding
- Hemoglobin and anemia status
- Previous C-sections or uterine surgery
- Signs of placenta accreta spectrum
- Baby’s position
- Gestational age
- Distance from your home to the hospital
- Availability of blood products and specialist care
If placenta previa persists and a cesarean is planned, your obstetric team can prepare for the possibility of significant bleeding.
This planning is one reason why diagnosing placenta previa before labor is so important.
Can Placenta Previa Affect the Baby?
Placenta previa does not usually mean that the baby has a structural abnormality.
The main concerns are bleeding and preterm birth rather than a fetal birth defect.
If significant bleeding occurs, an early delivery may become necessary. Babies born prematurely can have complications related to prematurity, which is why doctors try to balance the risks of continuing the pregnancy against the risks of an early delivery.
If there is no bleeding and the pregnancy is otherwise stable, your healthcare team may continue monitoring until the safest time for delivery.
The Bottom Line
Placenta previa on ultrasound does not always mean something is immediately wrong.
If it is found around the 20-week anatomy scan, many cases will resolve as pregnancy progresses. Approximately 9 out of 10 women with a low-lying placenta at the 20-week scan will no longer have a low-lying placenta at follow-up.
The important questions are:
Is the placenta covering the cervix?
How far is the placental edge from the internal cervical opening?
Is there vaginal bleeding?
Does the placenta remain low later in pregnancy?
If true placenta previa persists near delivery, a C-section is generally recommended because the placenta can block the birth canal and labor can cause serious bleeding. For stable placenta previa without other complications, SMFM/ACOG-endorsed guidance recommends planned delivery at 36 0/7–37 6/7 weeks.
If you have placenta previa and experience any vaginal bleeding, contact your healthcare provider or seek urgent medical care rather than waiting for your next scheduled appointment.
This article is for educational purposes and does not replace individualized medical advice. If your ultrasound report mentions placenta previa or a low-lying placenta, discuss the exact placental location, distance from the cervix and follow-up plan with your OB-GYN or maternal-fetal medicine specialist.
