Green’s Dictionary of Slang

(Aus.) pregnant.

1951		Cusack & James Come in Spinner (1960) 226: Guinea’s face lighted with unholy glee. ‘A Parker prego? Did I hear right?’.
1971		Guardian 27 May 13/7: Uncommon in print, but familiar in speech, is preggo (pregnant).
1989		H. Rawson Dict. of Invective (1991) 309: Mothers-to-be generally were described on formal occasions as [...] preggers (or preg, preggo and preggy).