Placenta praevia is a condition that occurs when the placenta is located in the lower part of the woman's uterus, in the way of the cervix.
It is not uncommon for a woman to have a low lying placenta, which is often diagnosed at the routine 20 week scan. In most cases, it usually moves upwards and out of the way of the cervix.
However, if it fails to move, there is a risk of major bleeding from 30 weeks onwards. The reason for bleeding during pregnancy is not fully understood, although bleeding during labour is attributed to the placenta lying over the cervix and detaching from the uterine wall as contractions progress. A blood transfusion may be necessary if the bleeding is excessive.
As well as diagnosis via ultrasound and the classic symptom of heavy bleeding, placenta praevia may also be suspected if the baby's head fails to engage. This is because the position of the placenta prevents the baby from adopting the head-down position. Instead, the baby will lie in a variety of positions, which is known as an 'unstable lie'.
Risk factors for developing placenta praevia include a previous history of the condition; a previous caesarean; and twin, or multiple, pregnancy.
There is no treatment for placenta praevia, other than waiting to see if the placenta moves. If the placenta fails to rise, a vaginal birth is not usually possible and a caesarean section will be required.