Low-lying placenta (placenta praevia)

The placenta is your baby’s support system in the womb. If the placenta doesn’t work properly, your baby is at risk of health problems.

The placenta processes your baby’s nutrients, waste and oxygen. 

In most pregnancies the placenta attaches to the side of the womb but for some women the placenta attaches lower down and may cover a part or all of the cervix (entrance to the womb). This is called low-lying placenta or placenta praevia.

This often shows up in early ultrasound scans when it is called low-lying placenta. In 90% of cases, the placenta moves upwards as the womb grows. For some women, however, the placenta continues to lie in the lower part of the uterus after 20 weeks. This affects 1 in 200 births.

When this happens it is called placenta praevia. If the placenta covers the cervix, this is known as major placenta praevia.

A note on the difference between placenta praevia and anterior placenta

People sometimes think that low-lying placenta is linked to having an anterior placenta but this is not correct. Anterior placenta simply means the placenta is on the front (belly side) of the womb rather than attached to the back of the womb. Having an anterior placenta is normal and does not cause complications. 

How would I know if I had placenta praevia?

Placenta praevia is usually spotted during your normal ultrasound scans but other signs are:

  • painless vaginal bleeding in the second or third trimester
  • baby in an unusual position in the second or third trimester, breech for example or failure of the head to engage in the pelvis just before labour starts.

If you have any bleeding, contractions or pain in pregnancy you should always contact the hospital.

You may have a transvaginal scan (where the probe is placed inside the vagina) to confirm whether you have placenta praevia.

The risk factors for placenta praevia

In many cases the cause of placenta praevia is unknown but the following risk factors are associated with the condition:

  • Maternal age: it is more common in women who are over 40 years
  • Previous caesarean section (the risk increases slightly with each one)
  • IVF
  • Previous placenta praevia
  • Multiple pregnancies
  • Smoking
  • Previous abortion or surgery in the womb

Treatment for placenta praevia

You will have extra scans if you are found to have a low-lying placenta in a routine scan.

An extra scan is recommended at 32 weeks if:

  • • you had a caesarean section before
    • your placenta covers the cervix (major placenta praevia)
    • your placenta is in an anterior position (at the front of the womb) and you have had a previous caesarean section.

If your placenta does not cover the cervix and you have no bleeding during your pregnancy, your extra scan should happen around 36 weeks instead. 

There is a risk that you may bleed in the second half of pregnancy (sometimes caused by having sex). Bleeding from placenta praevia can be heavy. If you bleed contact your hospital immediately.

If you have major placenta praevia (the placenta covers the cervix) you may need to stay in hospital after 34 weeks of pregnancy even if you have had no symptoms. There is a small risk that you could bleed suddenly and severely, which may mean that you need an urgent caesarean section. If you are bleeding the baby may need to be delivered.

Placenta praevia is linked to spontaneous preterm delivery and PPROM (waters breaking early), a major cause of premature labour.

The birth

Your obstetrician and / or midwife is likely recommend delivery in hospital, rather than a home setting, and caesarean section might be considered the safest option if the placenta is less than 2cm away from the internal os (the part of the cervix that opens into the uterus).

If you have major placenta praevia (where it covers the cervix) or if the placenta is very close to the cervix you will need to have a caesarean section because the placenta lies in the way of your baby being born.

Read more about having a caesarean section

Sources

  1. RCOG (2011) Information for you: A low-lying placenta (placenta praevia) after 20 weeks, Royal College of Obstetricians and Gynaecologists
  2. RCOG (2011) Placenta praevia, placenta praevia accreta and vasa praevia: diagnosis and management, Green-top Guideline No 27, Royal College of Obstetricians and Gynaecologists
  3. BMJ Best Practice (accessed Oct 2016) http://bestpractice.bmj.com/best-practice/monograph/667/diagnosis/history-and-examination.html
  4. BMJ Best Practice (accessed Oct 2016) http://bestpractice.bmj.com/best-practice/monograph/667/basics/aetiology.html
  5. J David, Steer P et al (2010) High risk pregnancy, management options, Elsevier Saunders
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Last reviewed on September 12th, 2016. Next review date September 12th, 2016.

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