There's a new advert appeared on TV in this week with a small amount of belly inflation in it (but well executed)
It's for a well known make of squash. The jist is that kids grow up fast, and it has a couple of parents playing with a baby in the garden, and the baby quickly grows to adulthood as the advert goes on.
Anyway, at the end a wife appears alongside the son and becomes pregnant in double-quick time. As I say, it's only slight, it's not all the way to nine-months, but it is pretty well executed.
https://www.youtube.com/watch?v=5HE_kJ08L1g