Low-Lying Placenta at 20 Weeks: Will It Move Up?

A low-lying placenta at 20 weeks means the placenta is sitting close to the cervix, the neck of the womb, on the mid-pregnancy scan. In most pregnancies it moves away as the womb grows: the Royal College of Obstetricians and Gynaecologists (RCOG) says 9 out of 10 women with a low-lying placenta at the 20-week scan no longer have one at follow-up.

RCOG and NHS hospital leaflets describe a placenta as low-lying when its edge is less than 20 mm from the cervix. Healthdirect Australia uses the same 2 cm line. When the placenta covers the cervix, it is called placenta praevia (placenta previa in US spelling). RCOG says only about 1 in 200 women overall still have placenta praevia at the end of pregnancy.

The placenta does not actually travel. As the lower part of the womb stretches and grows, the distance between the placenta and the cervix usually increases.

Bleeding from a low placenta is usually painless and can happen after sex, but it can still be serious. RCOG advises contacting the hospital straight away for any vaginal bleeding, contractions or pain, and notes a small risk of sudden heavy bleeding even without earlier warning. Call emergency services (999 in the UK, or your local number) if you have:

• Heavy bleeding, such as soaking a pad soon after putting it on.
• Bleeding with severe tummy pain or regular contractions.
• Feeling faint, dizzy or very unwell, or passing out.

Most do. The Irish health service (HSE) gives the same picture: about 9 in 10 women with a low-lying placenta find it has moved up by the third trimester and does not affect the birth.

Cambridge University Hospitals explains that the placenta's position is checked at the routine scan around 18 to 21 weeks, and that a vaginal scan may follow if it looks significantly low. As the womb grows, the placenta usually ends up further from the cervix.

These are figures for large groups of pregnancies, not a forecast for yours. The follow-up scan is what shows where your placenta sits later in pregnancy, and your team reads that result alongside any bleeding and your history.

RCOG says a follow-up scan is offered at 32 weeks if the placenta is low at 20 weeks, with a further scan at 36 weeks if it is still low. Healthdirect describes extra scans around 32 to 34 weeks. Local practice varies: one NHS leaflet offers a 32-week scan when the placenta covers the cervix and a 36-week scan when it is low but not covering it.

RCOG notes the follow-up scan may be done vaginally (a transvaginal scan), and that the length of the cervix may also be measured at the 32-week scan.

Go to your maternity unit as advised; do not wait for the next planned scan. The NHS says checks after bleeding may include an examination, an ultrasound scan, blood tests and a urine test, and you may be offered closer monitoring, which can mean staying in hospital. RCOG mentions that a speculum examination may be needed to check where the blood is coming from.

For placenta praevia, the HSE says early hospital admission may be recommended from 34 weeks, and possibly sooner if bleeding happens again. Healthdirect explains that heavy bleeding can sometimes mean the baby needs to be born early, and that steroid injections may be offered before 34 weeks to help the baby's lungs. Your team will explain which of these, if any, apply to you.

Many people with a low placenta at 20 weeks have no bleeding at all. Knowing the plan in advance simply means you know who to call and where to go.

Advice differs between sources, so ask your own team. RCOG notes that bleeding may happen after sex but gives no specific rule. For placenta praevia, the HSE advises avoiding sex to prevent more bleeding, and Healthdirect advises avoiding penetrative sex and tampons.

If your placenta is low but does not cover the cervix, ask whether any restriction applies to you and whether it changes after the follow-up scan.

Birth planning depends on the later scans. RCOG says that if the placenta edge is less than 20 mm from the cervix at the 36-week scan, a caesarean is the safest option; if it is more than 20 mm away, vaginal birth can be considered. A planned caesarean is usually at 36 to 37 weeks according to RCOG, while the HSE describes around 37 to 38 weeks, so timing varies by service and by whether there has been bleeding.

Because blood transfusion is more likely if heavy bleeding happens, RCOG and the HSE advise telling your team early if you would not accept a blood transfusion. RCOG also mentions eating well to avoid anaemia, with iron supplements if your team recommends them. If a blood test shows low red cells, our guide on low red blood cell count explains the results.

These questions can help you understand your own plan.

• How far is the placenta edge from the cervix, and does it cover it?
• When is my follow-up scan, and will it be transvaginal?
• Who do I call if I bleed, and what number do I use out of hours?
• Do I need to avoid sex, exercise or tampons, and until when?
• Is there anything in my history that changes my care or birth plan?
• What would make you plan a caesarean, and when would that be?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with a low placenta at 20 weeks described their follow-up scans and birth plans can help you think of questions for your appointment. Other people's outcomes cannot predict yours, and your maternity team's advice comes first.

If bleeding in early pregnancy was linked to a blood collection near the pregnancy sac, our guide on subchorionic hematoma covers that separate finding.