Scans to check the position of the placenta

Your sonographer will check and record the position of your placenta at your anomaly scan, which usually takes place between 18 weeks and 21 weeks of pregnancy. If your sonographer finds that your placenta is lying low in your uterus, you may be offered an ultrasound scan later in pregnancy, to see if it has moved.

Is a low-lying placenta a problem?

It depends on the stage of your pregnancy.

A placenta is low-lying if it’s touching or covering your cervix inside your uterus (womb).

If your placenta is found to be low-lying in a scan during the first half of your pregnancy, it’s usually nothing to worry about.

In most cases, the placenta moves up as the pregnancy progresses (NCT 2018).

But a low-lying placenta after 20 weeks of pregnancy (placenta praevia) can be a problem because:

• It may block the opening of your cervix inside your uterus (cervical os) and prevent your baby from being born vaginally (NHS 2018).
• There is a greater chance of bleeding during your pregnancy or your baby’s birth. The bleeding could be severe enough to put you and your baby at risk (RCOG 2018b).

Placenta praevia is more likely if you:

• are an older mum-to-be, especially if you're over 35 (Bakker 2018)
• are carrying twins or more (Bakker 2018)
• conceived using assisted reproductive technology (Bakker 2018, RCOG 2018a, b)
• smoke (Bakker 2018, RCOG 2018a, b)
• have had previous caesareans or other uterine surgeries, or abdominal injuries (Bakker 2018, RCOG 2018a)

Even given these quite common reasons, placenta praevia still affects only about one pregnancy in 200 at full term (NHS 2018, RCOG 2018a).

How will I know if I have a low-lying placenta?

If your placenta is too low, your sonographer will tell you at your 20-week anomaly scan. You may be offered another scan when you’re about 32 weeks pregnant to check the position of the placenta again (NCCWCH 2008). This may include a transvaginal scan, because it can get a more accurate picture of the placenta’s position (RCOG 2018b).

By this time, for nine out of ten women, the placenta will have moved up and out of the way of the cervix (NHS 2018).

If your placenta is still low, you may need another ultrasound scan a couple of weeks later, at around 36 weeks (RCOG 2018b).

Do I have to have all these scans?

Ultrasound scans during pregnancy aren’t compulsory, so it's up to you whether you have each one that’s offered. But an ultrasound scan is the best way to check where the placenta lies, and if it’s working properly to support your baby (RCOG 2018a).

Rest assured that ultrasound scans, including transvaginal scans, are considered to be perfectly safe for you and your baby (BMUS nd, RCOG 2018b).

What happens if my placenta is too close to my cervix?

Many women have a low placenta in the first trimester, but very few are still low at the end (RCOG 2018a, NCT 2018). So it’s unlikely that you’ll end up with placenta praevia.

If your anomaly scan shows that you have a low-lying placenta, try to keep your maternity notes with you at all times. This way, if you do have any bleeding, all of the information from your scan will be with you when you go into hospital.

Sometimes, placenta praevia causes painless, bright red bleeding from the vagina during the last three months of pregnancy.

The bleeding can be spontaneous or set off by something, for example, having sex (NCT 2018, RCOG 2018b). If you have these symptoms you should contact your doctor or midwife (NHS 2018).

If the placenta is still too low when your baby’s ready to be born, your doctor will discuss your options for giving birth with you. This is likely to include giving birth by caesarean section (NCT 2018).

Find out more about placenta praevia.