Board Thread:Spoilers!/@comment-6175354-20140126015900/@comment-5655661-20140126174022

Or WW of the West is actually Cora and Henry's daughter who got green skin as a price for Cora breaking a deal with Rumple. Since she was...deformed, it ruined Cora's dreams of making her a queen one day. So, she asked Rumple for help, he gave her baby Regina  in exchange for WW of the West. He raised her, taught her magic and sent her to OZ so she can bring him ruby slippers.

Resume: Regina is not Cora's daughter. Rumple already said that Regina didn't look like Cora at all (maybe because she's not her daughter) and that when he saw Regina for the first time she was "more portable" implying that he held her in his arms when she was a child.