Though there is such a thing as a real [spoiler]premature baby.
[/spoiler]
And one thing that writers love to do is put an alternate spin on what's real just to keep the audience guessing until the actual truth comes out, which in this case most likely would have been that [spoiler]David was indeed Roger's son.[/spoiler]
We especially should believe that when there was never any real evidence otherwise beyond Roger's remark (and he and Laura exchanged barbs with each other as often as most other people exchange pleasantries) and what Laura may have hinted to Burke (but Laura was a master manipulator when it came to Burke because all she was interested in was getting him to do her dirty work - she would have implied anything and everything if she thought it would motivate Burke, so anything she may have done with him is highly suspect).