Low-Lying Placenta Follow-Up Scans

Low-lying at 20 weeks is a finding, not a diagnosis

Most women told their placenta is low at the 20-week scan hear the words placenta praevia somewhere in the same conversation and leave frightened. The two are not the same thing at that stage, and the odds are strongly in your favour.

RCOG's patient information puts the numbers directly: "9 out of 10 women with a low-lying placenta at their 20-week scan will no longer have a low-lying placenta when they have their follow-up scan, and only 1 in 200 women overall will have placenta praevia at the end of their pregnancy."

The mechanism is not that the placenta migrates. The lower part of the uterus grows and stretches as pregnancy advances, carrying the placenta upwards and away from the cervix. RCOG describes it as the placenta moving "upwards and out of the way as the uterus grows during pregnancy".

The definitions your notes are using

The terms have specific meanings, and knowing which one is written in your notes tells you a great deal. RCOG defines the condition as "known as low-lying placenta if the placenta is less than 20 mm from the cervix or as placenta praevia if the placenta completely covers the cervix".

So "low-lying" and "praevia" are different findings with different implications, and a placenta described as low-lying at 20 weeks is in the group where nine out of ten resolve. Placental position is checked at the 20-week appointment as good clinical practice rather than as part of the national screening programme itself.

The scan schedule

RCOG's patient information sets out the pathway: "If your placenta is low lying at your 20-week scan, you will be offered a follow-up scan at 32 weeks of pregnancy to see whether it is still low lying. This may include a transvaginal scan. You should be offered a further ultrasound scan at 36 weeks if your placenta is still low lying."

Two things about that are worth expecting. The gap to 32 weeks is long, and it is deliberate — rescanning earlier mostly produces findings that would have resolved anyway. And the transvaginal scan, where the probe is placed inside the vagina, is the accurate way to measure the distance from the placental edge to the cervix. RCOG confirms it "is safe for both you and your baby and it may be used towards the end of your pregnancy to check exactly where your placenta is lying". It does not cause bleeding, which is the fear most women have about it.

Something else may be measured at the same appointment: "The length of your cervix may be measured at your 32-week scan to predict whether you may go into labour early and whether you are at increased risk of bleeding."

What the 36-week scan decides

This is the scan that determines how your baby is born, and the threshold is a single number. RCOG: "If the edge of your placenta is less than 20 mm from the entrance to the cervix on your scan at 36 weeks, a caesarean will be the safest way for you to give birth. If the placenta is further than 20 mm from your cervix you can choose to have a vaginal birth."

Where a caesarean is planned, "unless you have heavy or recurrent bleeding, your caesarean will usually take place between 36 and 37 weeks", and it should happen with "a senior obstetrician and anaesthetist" present, in a hospital equipped to manage heavy bleeding.

Who is less likely to resolve

RCOG notes that "if you have previously had a baby by caesarean, the placenta is less likely to move upwards", and that placenta praevia is more common after one or more previous caesarean births, after fertility treatment, and in women who smoke.

Previous caesarean matters for a second reason. RCOG's guideline on placenta praevia and placenta accreta notes that rates of both conditions have increased and are likely to keep increasing "as a result of rising rates of caesarean births and the use of assisted reproductive technology". Where a low placenta sits over a previous caesarean scar, teams look specifically for placenta accreta spectrum, which is a different and more serious problem.

What to do between now and the rescan

The instruction that matters is about bleeding. RCOG: "If you know you have a low-lying placenta, you should contact the hospital straight away if you have any vaginal bleeding, contractions or pain." Bleeding from a low placenta "is usually painless and may occur after having sex", and can be heavy without warning.

You will usually be advised about sex and about strenuous activity; advice differs between units and by how low the placenta is, so ask for yours rather than assuming. RCOG also advises trying "to avoid becoming anaemic during pregnancy by having a healthy diet and by taking iron supplements if recommended", because your haemoglobin is what carries you through any bleeding.

If the placenta is still low later on, there is a raised chance of the baby arriving before 37 weeks, and RCOG notes you may be offered "a course of steroid injections between 34 and 36 weeks of pregnancy to help your baby to become more mature".

The appointment to check exists

The single most useful thing you can do is confirm the 32-week scan is actually booked before you leave the 20-week appointment, and check again at your next antenatal contact. A follow-up scan that was intended but never booked is the most common way this pathway fails, and the whole point of the finding is the rescan.