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. This article covers:
A low-lying placenta is one where the placenta is lying in the lower part of the uterus near, or over, the opening of the womb (cervix). Most low-lying placentas are near but not covering the cervix. If your placenta is partially or completely blocking the womb opening at the end of pregnancy, this is known as known as ‘placenta praevia’ (pronounced ‘preev-ee-ah’). In these cases, your baby will need to be born by caesarean section.
However, a low-lying placenta in early or mid-pregnancy will most often have moved out of the way by late pregnancy.
During labour the cervix opens to let the baby move down into the vagina and be born. If the placenta overlaps the cervix, 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.
At your 18-21-week ultrasound scan your placenta’s position will be recorded. If your placenta is significantly low, you’ll be offered an extra scan to check again, usually at about 32 weeks.
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 around 1 in 600 had placenta praevia at the time of birth. In most cases the placenta, even if relatively low in the uterus, will not be affected by the cervix opening during labour.
As your pregnancy progresses, the uterus grows, particularly the lower part of the uterus, so the placenta moves with the growing uterus away from the cervix.
It does seem that screening by ultrasound in mid-pregnancy is often not very accurate and low-lying placenta may be over-diagnosed.
First, if the placenta is not actually covering the cervix 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’.
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.
In England and Wales, the Antenatal Care guideline recommends that as ‘most low-lying placentas detected at the anomaly scan will have resolved by the time the baby is born, only a woman whose placenta extends over the internal cervical os should be offered another transabdominal [from the abdomen] scan at 32 weeks.
If the transabdominal scan is unclear (at 32 weeks), you should be offered a scan via the vagina, according to these guidelines. An ultrasound scan through the vagina is more accurate in diagnosing the position of the placenta than a scan taken from the abdomen.
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 opening to the womb.
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.
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 opening, your obstetrician will advise you to have a caesarean section 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.
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.
If placenta praevia occurs following a previous caesarean, there is an increased risk of complications so a consultant obstetrician and anaesthetist should be present at the birth. ‘Placenta accreta’ may occur – where the placenta grows deeply into the wall of the uterus and will not separate normally. A detailed imaging scan should be performed in pregnancy to rule this out.
In severe cases of placenta accreta, a hysterectomy may be needed to stop very heavy bleeding at the birth.
NCT's helpline offers practical and emotional support in all areas of pregnancy, birth and early parenthood: 0300 330 0700. We also offer antenatal courses which are a great way to find out more about birth, labour and life with a new baby.
NHS choices offers information on problems with the placenta in pregnancy.
The UK has an NSC policy on Placenta praevia screening in pregnancy.