During last night's episode, I remembered that Danielle was already pregnant before she came upon the island, and never remembered her saying anything about someone named Ben. So, I looked on
Main Page - Lostpedia and did some researching. Here is what I found about Danielle:
"Danielle Rousseau led a science expedition around the Pacific along with her lover Robert. The two were very much in love, and Rousseau carried in her possession a music box he had once given to her. During one science voyage, three days out from Fiji, a seven-months pregnant Rousseau picked up a distress signal broadcasting the Numbers. Changing course to investigate, a freak storm caught the ship in its wake, causing it to crash on the Island."
Interesting. Obviously we're not going to be able to figure this out right now. But maybe Alex is just meant to think that Ben is her father?