Green’s Dictionary of Slang

(Aus.) to become pregnant after one’s first act of sexual intercourse, esp. on one’s marriage night.

c.1943		S .J. Baker Aus. Vulgarisms [t/s] 10: hole out in one: (Said of a girl) To become pregnant as a consequence of her first amour.
1955		N. Pulliam I Travelled a Lonely Land (1957) 234/1: hole out in one – a gal who makes a bad mistake, gets in trouble the first time she steps out.
1980		Maledicta IV:2 (Winter) 199: Holed out in one is pregnant after only one sexual act.