Are you mixing names? Because I'm pretty sure Gwen can't get pregnant.
But that aside.
Girls will have the "something is preventing them from getting pregnant" dialogue when:
A) They don't love you enough (love 4)
B) They're already pregnant
C) Girl already has a kid in the daycare
D) You have not been to the nexus
Usually for most people it's B, they don't realise she's already preggernet.