Green’s Dictionary of Slang

belly up adj.1

[the shape of her stomach]

of a woman, pregnant.

[UK]Barrère & Leland Dict. of Sl., Jargon and Cant.
[UK]Farmer & Henley Sl. and Its Analogues.
[UK]Partridge DSUE (1984) 68/2: C.17–early 20.