Michael Phelps -- I Knocked Up My Fiancee!

Turns out, Michael Phelps has got some strong little swimmers .... 'cause the Olympic legend just announced his fiancee is PREGGO!!

Phelps and Nicole Johnson just broke the news ... saying they're 12 weeks pregnant with a baby BOY!

They've already got the tiny speedos ready for the kid ... so, clear a spot in the '36 Olympics!

BTW, Phelps and Johnson got engaged back in February. They had been dating off and on since 2007. No word on when they plan to get married.

As for baby Phelps ... the kid is due around July -- right before the '16 Olympics.

Congrats!