Info Centre - Low-lying placenta

Low-lying placenta

If you have had an ultrasound scan in pregnancy, you may have been informed that you have a low-lying placenta. This sheet provides some information about this condition, and questions you can ask your doctor or your midwife.

What is the placenta?

The placenta is your baby’s ‘life support’ system during pregnancy. It is shaped a bit like a pancake attached to the inside of your womb (uterus), and your baby’s umbilical cord emerges from the centre. The cord is like a lifeline with blood going in two directions.

The uterus and the placenta have a fine membrane separating them, so that the blood of the mother does not mix with the baby’s blood but oxygen and nourishment are passed from the mother to the baby, and waste molecules are passed back. When the placenta separates after birth, there can be bleeding from the placental site. Normally the separation does not occur until after the baby has been born, and strong contractions close the blood vessels in the uterus, minimising the loss of blood.

What is a low-lying placenta?

A low-lying placenta is one where the placenta is lying in the lower part of the uterus near, or over, the cervical os (the opening of the womb). Most low-lying placentas are near but not covering the os. If your placenta is partially or completely covering the cervical os at the end of pregnancy, your baby will need to be born by caesarean section. When this happens, it is known as ‘placenta praevia’ (pronounced ‘preev-ee-ah’). Low-lying placenta in early or mid pregnancy is a very different condition from placenta praevia later on.

Why is placenta praevia at the end of pregnancy a problem?

During labour the cervix opens to let the baby move down into the vagina and be born. If the placenta overlaps the cervical os, the placenta will begin to separate as the cervix opens, causing bleeding, usually from the mother. Bleeding can be extensive and can occasionally lead to shock if it is not treated promptly. A blood transfusion may be needed.

If my placenta is low-lying in the middle of pregnancy, could this change?

Yes. The vast majority of women who have a low-lying placenta in the middle of pregnancy will not be affected in the later weeks. At the end of pregnancy, only a tiny percentage of women have the condition. One study of 6,428 women found that only 0.16 per cent of women (around 1 in 600) had placenta praevia at the time of birth.1 In most cases the placenta, even if relatively low in the uterus, will not be affected by the cervix opening during labour.

How accurate are scans to identify a low-lying placenta at around 20 weeks?

It does seem that screening by ultrasound in mid-pregnancy is often not very accurate and low-lying placenta is over-diagnosed. First, if the placenta is not actually covering the cervical os at 20 weeks of pregnancy, it is unlikely that there will be a problem later in the pregnancy. Second, the report of evidence used to inform the Antenatal Care guideline for the NHS says, ‘Many placentas that appear to cover the cervical os (the opening in the cervix inside the womb) in the second trimester will not cover the os at term’.²  This is partly because as the weeks of pregnancy go by and the lower part of the uterus grows and stretches, the placenta appears to move away from the cervix.

An ultrasound scan through the vagina is more accurate in diagnosing the position of the placenta than a scan taken from the abdomen, as the internal os and edge of the placenta can be seen more clearly (see ‘What if I have a low-lying placenta in the middle of pregnancy?’ below).

Why does the position of the placenta change?

The earlier a scan is carried out during pregnancy, the more likely the placenta is to appear close to the cervical os. This is because the lower segment of the uterus is still underdeveloped.³ As the pregnancy progresses, the uterus grows, particularly the lower part of the uterus, so the placenta moves with the growing uterus away from the cervix.

What if I have a low-lying placenta in the middle of pregnancy?

What are the risks of placenta praevia?

Placenta praevia is associated with a higher rate of pregnancy complications, including separation of the placenta, bleeding and reduced growth for the baby, so additional monitoring will be offered to women who do have a placenta that overlaps the cervical os.

Bleeding during pregnancy is fairly common; it is more likely to occur in women with a low-lying placenta. If you have any bleeding, your midwife, GP or obstetrician can advise you. If you have any concerns call them and explain your symptoms.

What if I have placenta praevia at the end of pregnancy?

If you have placenta praevia at the end of pregnancy, you may be advised to rest in bed and avoid having sex and orgasms. You may also be advised to spend the last few weeks of pregnancy in hospital, particularly if you have had any bleeding, so that you can have a caesarean section straightaway, if necessary. Home-based care is also possible if the hospital can be reached easily. Medical care is needed if you have bleeding or show any signs of going into labour.

If the edge of the placenta at the end of pregnancy is less than 2cm from the cervical os, your obstetrician will advise you to have a caesarean section³ to prevent major bleeding and to ensure that your baby can be born safely. If the placenta is lying towards the back, a slightly wider margin (3cm) may be recommended.³

Are some women more at risk than others?

The chance of having placenta praevia is greater for:

• women who have previously had placenta praevia
• women who have had a previous caesarean
• older women
• women expecting twins
• women who have had several babies
• women who smoke
• women who use cocaine
• women who have had a miscarriage.

absolutely.³ If the placenta is thought to be possibly involve the old scar, there is an increased risk of significant complications such as bleeding and a hysterectomy may be needed to control this at birth. The hospital may want to make preparations for the delivery and a consultant obstetrician and anaesthetist should be present at the birth.³