Loosening of the placenta
Before the baby is born, the placenta can get loosened from the wall of the uterus as a result of a haemorrhage in the zone. The symptoms are strong pain in the abdomen together with an occasionally dark haemorrhage through the vagina.
It is associated to pre-eclampsia (arterial hypertension in some pregnant women) and usually happens in the third trimester. It can get to be serious, since the loosened part of placenta stops contributing oxygen to the baby, so once the problem is diagnosed, a caesarean section has to be made urgently.