Mathematical calculation to determine the time a female got knocked up. Most often used to narrow down possible biological fathers of the baby. (date expecting- 9).
Guy 1 and Guy 2 chatting:
Guy 1: "Hey, did you hear Jullie is gonna have a baby in March?"
Guy 2: (inwardly calculates pregnancy math.....)
Guy 2: "Naw, I didnt know that. But that ought to narrow down the possible dads to about a dozen guys she was with last July"