OEIS complex, also known as cloacal exstrophy, is the most severe birth defect within the exstrophy-epispadias complex. It is characterized by Omphalocele, Exstrophy, Imperforate anus and Spinal defects. A child with this condition will have the bladder and a portion of the intestines exposed outside the abdomen with the bony pelvis open like a book. In males, the penis is either flat and short or sometimes split. In females, the clitoris is split and there may be two vaginal openings. Also, frequently the intestine is short and the anus is not open (anus imperforate). There is a high association with other birth defects, especially spina bifida, which occurs in up to 75% of cases. Omphalocele, a defect of the abdominal wall in the region of the umbilicus, is also common, as are kidney abnormalities. The defect occurs when the baby is developing inside the uterus during the first trimester of pregnancy, and seems to be due to the rupture of a tissue known as the cloacal membrane which results in the abnormal development of the abdominal wall and other malformations. The timing of the rupture determines the severity of the disorder. The cause for this malformation is unknown.