Placenta previa

The placenta is an organ that develops in the uterus during pregnancy. It supplies the baby with oxygen and nutrients through the umbilical cord.

Placenta previa occurs when the placenta lies over the cervix. The cervix is the opening between the uterus and vagina. The placenta may cover part or all of this opening.

A low-lying placenta is close to the cervix but does not cover it. It is related to placenta previa, but the two terms do not always mean the same thing. Your ultrasound report may describe the distance between the placental edge and the cervix.

Placenta previa matters because the cervix begins to soften, shorten, and open near the end of pregnancy. This can separate part of the placenta from the uterus and cause bleeding. A placenta that continues to cover the cervix also blocks the usual route for vaginal birth.

Many low placentas found during the middle of pregnancy are no longer close to the cervix later on. The placenta does not crawl across the uterus. Instead, the lower uterus stretches as pregnancy progresses, increasing the distance between the placenta and cervix.

The main symptom is vaginal bleeding after about halfway through pregnancy. The blood is often bright red, and the bleeding may be painless. It can begin suddenly, stop, and then return later.

Bleeding may happen after sex or contractions, but it can also happen without an obvious trigger. The amount can range from light spotting to heavy bleeding.

Some people have uterine tightening or contractions with the bleeding. Pain is not typical of placenta previa, but its presence does not rule it out. Painful bleeding can have other causes that also need urgent assessment.

Many people have no symptoms. In these cases, placenta previa or a low-lying placenta is first seen during a routine pregnancy ultrasound.

Any vaginal bleeding in the second half of pregnancy should be assessed promptly. It is not possible to confirm the cause based only on the colour, amount, or presence of pain.

Placenta previa begins when the fertilized egg implants low in the uterus and the placenta develops near or over the cervix. Why this happens in a particular pregnancy is often not known.

It is more likely in people who have:

• Had placenta previa in a previous pregnancy
• Had one or more cesarean births
• Had other surgery involving the uterus
• Been pregnant several times
• A pregnancy with more than one baby
• Become pregnant through assisted reproductive treatment
• Smoked or used cocaine during pregnancy
• Scarring or structural differences inside the uterus

Having a risk factor does not mean placenta previa will occur. It can also develop without any known risk factors.

Previous cesarean birth is especially important because a placenta overlying a cesarean scar has a higher chance of attaching too deeply. This is called placenta accreta spectrum.

Placenta previa is confirmed with ultrasound. It may first be suspected during the routine mid-pregnancy anatomy scan or after an episode of bleeding.

An ultrasound over the abdomen can show the general placental position. A transvaginal ultrasound, in which a narrow ultrasound probe is gently placed inside the vagina, gives a more accurate view of the cervix and placental edge. It is considered safe when performed for this purpose and does not touch the placenta.

If the placenta is low at the mid-pregnancy scan, another ultrasound is usually arranged later, often at about 32 weeks. A further scan may be needed closer to birth if it remains low. Timing can differ according to the scan findings and local maternity practice.

Your care team may also check:

• Your blood count, to look for anemia after bleeding
• Your blood group and Rh status
• The baby’s heart rate and wellbeing
• Whether you are having contractions
• Signs of placenta accreta spectrum when relevant

A digital vaginal examination, where fingers are inserted to feel the cervix, can provoke severe bleeding if placenta previa is present. It is generally avoided until ultrasound has ruled out previa. A speculum examination, using an instrument that allows the vagina and cervix to be seen, may sometimes be performed in a setting prepared to manage bleeding.

Management is based on how much bleeding has occurred, the placental position, the stage of pregnancy, and the health of you and the baby.

If placenta previa is found on ultrasound but there has been no bleeding, care usually involves follow-up scans and a plan for where and how to give birth. You may be advised to seek help immediately if bleeding begins.

After a bleeding episode, assessment may include monitoring the baby, blood tests, and observation in hospital. Fluids or a blood transfusion may be needed if blood loss is substantial. If your blood type is Rh-negative, you may be offered anti-D immunoglobulin after bleeding to reduce the chance of developing antibodies that could affect the baby.

When early birth appears possible, corticosteroid medicine may be offered to help the baby’s lungs mature. Other treatment may be considered according to the stage of pregnancy and whether contractions are occurring.

Some people can return home after bleeding has stopped. Others are advised to remain in hospital because of repeated bleeding, their distance from emergency care, or other pregnancy concerns.

Strict bed rest is not routinely helpful and can increase the risk of blood clots and loss of strength. Advice about sex, exercise, travel, and work should be based on your bleeding history and scan findings. After bleeding, avoiding activities that could trigger further bleeding may be recommended.

A placenta that still covers the cervix requires cesarean birth. For an uncomplicated persistent placenta previa, this is often planned at around 36 to 37 weeks, before labor is likely to begin. Earlier birth may be needed if there is heavy or repeated bleeding, labor starts, or either you or the baby becomes unwell.

A low-lying placenta that no longer covers the cervix does not always require cesarean birth. Whether vaginal birth is reasonable depends on its exact distance from the cervix, your bleeding history, the baby’s position, and the maternity unit’s experience.

The main complication is bleeding before, during, or shortly after birth. Severe bleeding can require an urgent cesarean birth, blood transfusion, or additional procedures to control blood loss. Removal of the uterus, called a hysterectomy, is rarely required for bleeding that cannot otherwise be controlled.

Other possible complications include:

• Anemia from repeated blood loss
• Preterm birth, meaning birth before 37 weeks
• A baby lying sideways or bottom-first because the placenta occupies the lower uterus
• Heavy bleeding after birth
• Placenta accreta spectrum

Placenta accreta spectrum means the placenta is attached too deeply to the uterine wall. It is more likely when placenta previa lies over a previous cesarean scar. If scans suggest this condition, birth is usually planned in a specialist hospital with an experienced team and access to blood transfusion.

Most babies are not directly harmed by the placenta covering the cervix. The main risk to the baby is being born early if bleeding makes delivery necessary.

Placenta previa usually cannot be prevented. There is no reliable action that can change where a placenta has already implanted.

Not smoking and avoiding cocaine during pregnancy reduce several pregnancy risks and may also lower the chance of placental problems. However, placenta previa can still occur in people who have never used either substance.

Avoiding medically unnecessary uterine surgery may reduce scarring, but cesarean birth and other procedures are sometimes the safest or necessary option. If you have had placenta previa or uterine surgery, tell your maternity team early in a future pregnancy so placental position can be checked.

A low placenta found at the mid-pregnancy scan often moves clear of the cervix as the uterus grows. Follow-up imaging shows whether this has happened. A placenta covering the cervix more fully or remaining low later in pregnancy is less likely to clear before birth.

Between appointments, you may be able to continue many usual activities if you have not had bleeding. Your maternity team may suggest changes based on your circumstances. These can include staying within practical reach of a hospital and making plans for transport and childcare in case bleeding begins.

Repeated bleeding can make pregnancy feel uncertain. It may lead to several hospital assessments or an inpatient stay. Ask for a clear plan explaining which maternity unit to contact, where to go, and whether you should call an ambulance for heavy bleeding.

If the placenta remains over the cervix, the main long-term change is the need for a planned cesarean birth. Placenta previa itself ends after the placenta is delivered. Recovery then depends on the amount of bleeding, whether the birth was planned or urgent, and whether complications such as placenta accreta were present.

Placenta previa can happen again in a later pregnancy, although it does not occur in every future pregnancy. An early review of your pregnancy history and careful assessment of placental position are appropriate next time.

Contact your maternity assessment unit promptly for any vaginal bleeding during the second half of pregnancy, even if it is light, painless, or stops on its own. Do not wait for a routine appointment.

Call emergency services if:

• Bleeding is heavy, ongoing, or rapidly soaking a pad
• You feel faint, dizzy, very weak, confused, or short of breath
• You have severe abdominal pain or frequent contractions
• You collapse or cannot travel safely
• Your baby’s usual movements are reduced alongside bleeding

Use an ambulance rather than driving yourself when bleeding is heavy or you feel unwell. Until help arrives, lie on your side if comfortable and follow instructions from emergency staff.

Routine follow-up is appropriate when you have no bleeding or new symptoms and are attending planned scans or discussions about birth. Contact your maternity team sooner if you develop contractions, fluid leakage, or a change in your baby’s usual movements.

Can placenta previa go away on its own?

It can clear before birth, especially when the placenta is only low at the mid-pregnancy scan. As the lower uterus stretches, the placental edge may end up farther from the cervix. Follow-up ultrasound is needed to confirm its position.

Can I have a vaginal birth with placenta previa?

Not if the placenta still covers the cervical opening. In that situation, cesarean birth is needed because labor could cause severe bleeding. Vaginal birth may be possible with some low-lying placentas that do not cover the cervix.

Is a transvaginal ultrasound safe with placenta previa?

Yes. It is the most accurate way to measure the placental edge in relation to the cervix. The probe stays within the vagina and does not enter the cervix or touch the placenta.

Does placenta previa always cause bleeding?

No. Some people have no bleeding and learn about the condition during an ultrasound. Others have one or several episodes. The absence of earlier bleeding cannot predict with certainty what will happen later.

Do I need bed rest or to avoid sex?

Strict bed rest is not routinely recommended because it has no proven benefit and can cause harm. Advice about sex and physical activity is individualized, especially after bleeding. Follow the plan given by your maternity team.

Will placenta previa affect future pregnancies?

It raises the chance of placenta previa in another pregnancy, but recurrence is not certain. Previous cesarean birth or other uterine surgery may also affect where a future placenta attaches. Tell your maternity team about this history early in your next pregnancy.