Green’s Dictionary of Slang

fill in v.3

(Aus.) to make pregnant; thus filled in, pregnant.

[[US]Whip & Satirist of NY & Brooklyn (NY) 2 July n.p.: The Bedford street dentist [...] Report says there is other kind filling done there besides filling teeth].
[Aus]‘No. 35’ Argot in G. Simes DAUS (1993).