After delivery of an infant the placenta may occasionally fail to separate from the uterus, as the placenta may become adherent to the muscle of the uterus. This may cause serious haemorrhage that can lead to hysterectomy, serious maternal illness and maternal death. This condition is called the placenta accreta spectrum (PAS) and is becoming more frequent. The increased frequency of PAS has in turn, been attributed to the increasing rate of delivery by caesarean section. Previous gynaecological surgery has also been associated with the development of PAS, where the uterus has become scarred, but this is a much less common antecedent. However, PAS can exist in microscopic form also. The incidence of this condition is unclear as it is not been researched in a systematic way nor are the effects of the presence of microscopic PAS fully understood. The purpose of this project is to determine the incidence of microscopic PAS using a number of pre-specified pathological criteria. The clinical antecedents of those patients who have microscopic PAS will be identified and to see if there are any features during pregnancy that predispose to microscopic PAS and that may permit the identification of women at increased risk of major postpartum haemorrhage and other serious consequences in future pregnancies.