Placenta praevia means that the placenta it lies in front of - ‘praevia’ - the presenting part of the baby. In practical terms it means between the baby and the exit via the cervical canal. Mostly the placenta lies at the back of the womb or towards the top. In a small percentage of pregnancies it develops low down in the uterus and may obstruct the opening. Minor degrees of placenta praevia, detected of by ultrasound in the early stages of pregnancy, may correct themselves as the uterus grows, but anything more severe may cause antepartum haemorrhage, and will require delivery by Caesarean section.