I recently bought
Alien on DVD, and one of the special features is character dossiers. In Ripley's several of the entries note that she requested and was ultimately granted maternity leave. I forget whether or not the movies even mention what happened to her child, whether born or unborn. But if she was once pregnant (and this is a huge shot in the dark), perhaps Weyland-Yutani engineered a strain of Aliens to ultimately impregnate Ripley
. It's the only connection I can think of that fits that information.