zpghttp://www.oed.com/dictionary/zpg_n%3Ftab%3Dmeaning_and_useZPG, n.
First published 1986; not fully revised

ZPGnoun

  1. 1970–
    Zero population growth.
    1. 1970
      Of all the cries that have arisen, perhaps none is quite so superficially appealing, or so profound in its long-range social and economic implications, as zero population growth—‘ZPG’ in the slogans of the day.
      New York Times 3 June 61/6
    2. 1978
      This may have reduced the maximum possible number of children per female to around five, and it does not then require harsh assumptions about mortality rates to end up with ZPG.
      Nature 6 April 491/3

Originally published as part of the entry for Z, n.

Z, n. was first published in 1921; not fully revised.