Green’s Dictionary of Slang

fallen on adj.

pregnant.

1976

19761977197819791980198119821983

1984

1976		New Society 15 Jul. n.p.: As soon as a woman realised she had ‘fallen on’ or ‘got caught’ she would take action.
1984		Partridge DSUE (8th edn) 377/1: since ca. 1930.