I think she needs to be straddling my hard, throbbing cock. On top, bearing down, feeling my fat pulsating cock-head, pressing against her swollen, preggo, plugged cervix.

That would send her at first into orgasmic contractions, absolute bliss. The best follows, she has a whole-body orgasm, her pregnant belly rhythmically contracting on my big cock, while you watch, unable to move.