I think its hard to do this sort of thing in a comedy. That may sound obvious, but I remember during the finishing episodes of Only Fools and Horses (may or may not know wtf i'm on about) they did this. It wasn't that it was done insensitively or badly, its just very difficult to enjoy the jokes when they're amongst such serious stuff.
And my silence in that department spoke my answer :)
Although I do like the American soap approach of just filming their head for 6 months. Or.. would it be all that unfeasible for them to do a Monica gives in to food storyline for half a season? Better still, if she can get pregnant at the right time it'll be the finale before shes showing and the baby will be out before they restart filming. After all, theres only 9 or 10 left this season.
Well, here's a thought...how about instead of making it sad, Monica and Chandler have dreams (read: nightmares) about what the sprog will look like or about its personality (as in a combination of both of them and getting the worst of the bargain).
Then instead of talking to each other about it, (this part I'm not too sure about...they tell their buddies, who suggest they talk to each other's parents) or they could just be still in their secretive state, so they decide to talk to each other's parent's on their own...and this is where we finally meet Chandler's dad. The Gellar's can tell Chandler horror stories about what Monica was like as a little girl and the Bing's can argue about what they remember about Chandler.