A condition in which the placenta covers or partially covers the opening of the cervix into the uterus
In some pregnancies, the placenta is implanted lower down in the uterus and closer to the cervix than is normal. Although a placenta that is lying low will usually move upwards gradually as the uterus expands, the placenta may remain in this position and cover some or all of the opening of the cervix. This condition, known as placenta praevia, occurs in about 1 in 200 pregnancies and is more common in women over the age of 35. Placenta praevia causes up to 1 in 5 cases of vaginal bleeding that occurs after week 24 of pregnancy.
The severity of this condition is related to how much of the opening of the cervix is covered by the placenta. In marginal placenta praevia, the placenta lies low in the uterus and just reaches the edge of the cervix. When complete placenta praevia occurs, the whole of the cervix is covered. Symptoms caused by the condition vary, and mild cases may cause no adverse effects. In other cases, intermittent light to heavy vaginal bleeding occurs from week 24 of pregnancy onwards. Complete placenta praevia may cause severe bleeding that can be life-threatening to the mother and/or fetus. Women who have placenta praevia are at increased risk of developing postpartum haemorrhage because the lower part of the uterus may not be able to contract sufficiently to constrict the blood vessels of the uterus and stop bleeding after birth.
There is an increased risk of placenta praevia in women who have had several pregnancies or if the uterus has been scarred by previous surgery, such as a caesarean section. The placenta may also develop low in the uterus if there are noncancerous tumours present (see Fibroids). The risk of placenta praevia is increased in a multiple pregnancy because there may be more than one placenta or because the placenta may be larger than in a normal pregnancy (see Multiple pregnancy and its problems).
Usually, placenta praevia is detected after week 20 by routine ultrasound scanning (see Ultrasound scanning in pregnancy). If the placenta is lying low, you will be given follow-up scans to monitor its position. The problem may disappear because, in many cases, the placenta moves upwards and away from the cervix by week 32.
If the placenta remains low and you develop vaginal bleeding, you will be admitted to hospital. If you have light bleeding, you may need only bed rest, but if bleeding is heavy, you may need to have an emergency caesarean section and possibly a blood transfusion to replace the blood you have lost.
Even if there are no problems, when placenta praevia is complete you may be advised to stay in hospital for monitoring from about the 30th week of pregnancy because of the risk of bleeding. Your baby will then be delivered by caesarean section at 38 weeks. A caesarean section is usually necessary even if there is marginal placenta praevia because of the risk of haemorrhage.
With close monitoring, the pregnancy is usually successful. Placenta praevia may recur in a future pregnancy if an underlying cause cannot be treated.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.