When Tony Stark found out that one of his many one night stands was pregnant, he paid her off. During his playboy days, he wasn't interested in taking care of a baby, his or not. He wanted nothing to do with the girl or her child.
Money in hand, she found no reason to tell the billionaire that she was carrying twins, his twins. She simply flicked her copper curls over her shoulder and basked in the warmth of her new fortune.
Stark didn't hear about her again, not until five years later. After Afghanistan, Monaco, New York, Malibu, the Avengers. He got a call from social services. His daughter is unable to stay with her mother anymore, and he's her last chance before she's chucked into the system.