The placenta, which is of fetal origin, has over 100 highly vascular spots called cotyledons which interconnect with similar highly vascular spots on the uterus called caruncles. Together, these form a unit called the placentome. Retained fetal membranes occur when the cotyledons fail to separate from the caruncles. Fetal membranes are normally expelled within 3-8 hours after birth, and they become classed as 'retained' if they are still present 12 hours or more after birth. Duration of retention is highly dependent on uterine muscular contractions, which cease after 36 hours post-partum.