The diagnosis of placental abruption was based on
the following criteria, confirmed upon clinical examination by the attending physician: complete or partial separation of a normally implanted placenta occurring prior to delivery and confirmed at delivery by evidence of retroplacental bleeding, usually accompanied by abdominal pain. Information on the severity of abruption was not recorded on the medical charts. The diagnosis of placenta previa was based on the following criteria: partial or complete coverage of the internal cervical os by the placenta, usually associated with uterine bleeding and usually warranting cesarean delivery. Ah instances of placenta previa were confirmed by a positive ultrasound diagnosis, although a diagnosis based solely on ultrasound was not considered sufficient.