I've always thought of Fate as being the "dad," actually. Bardiche does call her "sir." So I just imagine Nanoha being pregnant and Fate just worrying her ass off and being all "NO DON'T GET UP I'LL GET THAT" and Nanoha's "Geez, Fate-chan, I'm not THAT helpless yet..."
Fate is the dad in her role for what she does... she travels for work, isn't always home, and works more than she's at home doing housework... but for how she interacts when she's at home she's the mom, she's the sweet and gentle one who's soft on the kids.
Nanoha might make Fate have the kid just to make sure she stays home all throughout the pregnancy!
Fate would probably insist on carrying the kid, actually, considering how Nanoha overworks herself nonstop no matter what she's doing, unless she's under constant supervision.
She had to be designated "unfit to return to duty" and all but put under house arrest so that she'd relax enough to let her body heal back to full capacity after the JS Incident, for cryin' aloud.
