it’s just pure speculation at this point. There have been no spoilers to say that Dan is the father. It’s just that Carly admitted that her affair was with a colleague at a medical convention many years ago. Then she comes to town and we find out that her and Dr. Dan were "old" friends back in the day, and that she (Carly) introduced Dan to his wife, Rebecca; so, in most of our just a little bit dirty, but mostly intuitive, minds it was automatically assumed….AH HA! No spoilers printed on this though.