chorionic villi attachment dictates the amount of haemorrhaging that can be expected. The chance of abnormal placentation increases in subsequent pregnancies, if there is scar tissue present from previous pregnancies. For example, previously undergoing a caesarean or placenta previa increases the likelihood of abnormal placentation, therefore increasing the chances of antepartum haemorrhaging. Placental abruption Placental abruption occurs when the placenta detaches from the endometrium. Detachment causes antepartum haemorrhaging at the location of abruption. Depending on the site of detachment, haemorrhaging may or may not be apparent. If abruption occurs behind the placenta where blood cannot escape through the cervix, blood will pool and form a retroplacental clot. Only when the site of detachment occurs on the side facing the cervical opening can the total amount of haemorrhaging be measured by vaginal bleeding. Using vaginal bleeding as a measurement of the severity of the placental abruption is therefore ineffective. The scale of haemorrhaging depends on the degree to which the placenta has separated from the uterine wall. In the case of partial placental separation, haemorrhaging can be minor. However, in the case of total placental separation haemorrhaging will be major and emergency delivery will typically be the course of action. Placental abruption causes blood loss from the mother and loss of oxygen and nutrients to the placenta occasionally leading to preterm labour. Other causes of placental abruption can be abdominal trauma or sudden decompression of amniotic fluid, however it is not uncommon for the cause of placental abruption to be unknown. Vasa praevia Vasa praevia is the presence of unprotected foetal blood vessels running along the placenta and over the internal cervical opening. Vasa praevia is a very rare, presenting only 4:10,000 cases from the largest study of the condition. Risks of antepartum bleeding due to vasa praevia greatly increase during the third trimester of pregnancy during cervical dilation or placenta praevia. Vessel rupture is very likely in the event of a membranous rupture as foetal blood vessels aren't protected by the umbilical cord of the placenta. In the event of foetal vessel rupture, antepartum haemorrhaging occurs however blood is lost from the foetal blood supply. If the foetus is developed enough caesarean sections are often recommended. Abnormal placental shape Circumvallate placenta A circumvallate placenta refers to when the foetal membrane wraps twice around, over the foetal side around the edge of the placenta. This is to compensate for an undersized