a separation (or membrane) found in the uterus?!

Magin
Had my first trimester ultra sound last week. As they were roaming around up there I saw them keep looking in the same area over and over. She at that point voiced that there was a membrane separation. The nurse then said, "she said that they twins run in the family."  The doctor showed us the one baby, and then continued to look for what we were thinking was maybe another one. She didn't find one and didn't really give an explanation. 
Has anyone had this happen to them or know what it's all about?