Placenta praevia (placenta previa): symptoms, scans and how birth is planned

The placenta develops with the baby and attaches to the wall of the womb. It connects you and your baby, passing oxygen and nutrients from your blood to your baby's blood.

In early pregnancy it is common for the placenta to sit low. As the womb grows, the area where it is attached usually stretches upwards, away from the cervix (the neck of the womb). The NHS explains that if the placenta stays low, near to or covering the cervix, it may block the baby's way out.

The NHS uses two terms. Low-lying placenta means the placenta is less than 2 cm from the cervix, and placenta praevia means it completely covers the cervix. The RCOG gives the same threshold as 20 mm. Your scan report will say which applies to you.

Often there are no symptoms and the placenta is found to be low at the routine 20-week scan. When there are symptoms, the NHS says a low-lying placenta can be associated with painless, bright red bleeding from the vagina during the last 3 months of pregnancy.

The RCOG adds that bleeding from placenta praevia is usually painless and may occur after sex, and that placenta praevia may also be suspected if the baby is lying in an unusual position, such as bottom first (breech) or across the womb (transverse).

Even without earlier symptoms, the RCOG says there is a small risk of a sudden, heavy bleed that may mean an emergency caesarean.

The position of your placenta is recorded at the routine 20-week scan. If it is significantly low, the NHS says you will be offered an extra ultrasound scan later, usually at about 32 weeks.

The RCOG describes a follow-up scan at 32 weeks, which may include a transvaginal scan (a probe gently placed inside the vagina). It says this is safe for you and your baby and is how placenta praevia is confirmed. If the placenta is still low, a further scan at 36 weeks should be offered, and the length of the cervix may be measured at 32 weeks to help predict early labour and bleeding risk.

Usually, yes. The RCOG says that 9 out of 10 women with a low-lying placenta at their 20-week scan will no longer have a low-lying placenta at the follow-up scan, and only 1 in 200 women overall will have placenta praevia at the end of pregnancy.

Movement is less likely after a previous caesarean. The RCOG says the placenta is then less likely to move upwards.

The RCOG says placenta praevia is more common if you have had one or more previous caesarean births, if you had fertility treatment to become pregnant, or if you smoke.

Bleeding can be very heavy and put you and your baby at risk, according to the NHS. If this happens, contact your midwife or GP immediately. The RCOG says that if you know you have a low-lying placenta, contact the hospital straight away with any vaginal bleeding, contractions or pain. You can also call NHS 111 for advice, and call 999 if you feel seriously unwell or the bleeding is heavy.

• Any vaginal bleeding once a low placenta has been noted, even a small amount.

• Contractions or tightenings before term.

• Pain, particularly with bleeding.

The RCOG explains that placenta praevia increases the chance of the baby being born before 37 weeks, so steroid injections between 34 and 36 weeks may be offered to help the baby's lungs mature, and a medicine to slow contractions (tocolysis) may be offered if labour starts early.

The NHS says you may be advised to come into hospital at the end of pregnancy so that emergency treatment such as a blood transfusion can be given quickly if you bleed. The RCOG says additional care and any admission depend on your circumstances.

The RCOG advises trying to avoid anaemia with a healthy diet and iron supplements if recommended, with regular haemoglobin checks. If you could never accept a blood transfusion, tell your team as early in pregnancy as possible.

The NHS says that if the placenta is near or covering the cervix, a caesarean will be recommended. The RCOG says that if the edge of the placenta is less than 20 mm from the cervix at the 36-week scan, a caesarean is the safest way to give birth, whereas if it is further away you can choose a vaginal birth.

Unless there is heavy or recurrent bleeding, the RCOG says the caesarean usually takes place between 36 and 37 weeks, and earlier if you have bled. A senior obstetrician and anaesthetist should be present, and birth should be in a hospital that can care for heavy bleeding.

Bleeding during the caesarean may be heavier than average, and the RCOG says a hysterectomy may be needed if it cannot be controlled in other ways. You are also more likely to need a blood transfusion.

On the NHS, placenta position is recorded at the routine 20-week scan, with extra scans and obstetric-led care arranged if it is low. Some people have additional private scans, but the follow-up scans, hospital planning and caesarean timing should still be agreed with your NHS maternity team, because they know your full history.