Story Writing Discussion

Well, since I'm not a mom and the odds of me becoming one are almost nil, I'm not certain how best to answer your question.
Personally, I think the best thing I can say is to apply this question to your question... What type of story are you attempting to write? If it is a "vanilla" type story with warm, fuzzy memories, then make the lady of the story already pregnant.
However, if you're going for a harder edge to your story, then you again have to ask what type story you're trying to write. Are you trying to set up an incest/slave type situation to your family story? If so, start your mother off as getting pregnant during the course of the story. If you're trying for a rape/bondage style story, start her off pregnant.
In reality, you should be able to crossover either way, but experiment around and see which way makes it easier for the story style you want.
Rockdog