Normally the placenta attaches to the front, back or side of the uterus, but if it attaches to the lower part, it may cover the opening of the uterus, through the cervix to the outside. This is placenta praevia. It is more common in women who have had several pregnancies, and much more common in those who have had a caesarean section. Overall it occurs in one in every 150 pregnancies.
In the later stages of pregnancy, the cervix starts to dilate to allow the head of the baby to drop, prior to labour starting. If the placenta is over the opening, it will be damaged by the dilation of the cervix and the pressure from the baby’s head, and heavy bleeding may occur suddenly.
Placenta praevia may be suspected by the presence of a baby that is unusually high in the womb, and the position of the placenta can be seen accurately on an ultrasound scan. When diagnosed, the mother will be watched carefully, often in hospital, and about a month before the due date, a Caesarean section will be performed to remove both baby and placenta safely.
A bleeding placenta praevia can be a medical emergency, as quite torrential bleeding can occur which may threaten the lives of both mother and baby. The only treatment is an urgent Caesarean section.