Pregnant Man -- How I JACKED UP!

Former woman turned transgendered pregnant man Thomas Beatie didn't get ripped overnight -- in fact, he tells TMZ, it took LOTS of hard work to drop the baby weight ... and even some hormones.

Beatie -- who has given birth to three children -- called in to TMZ Live today, telling us, he owes his chiseled bod to good old-fashioned diet and exercise ... claiming he eats high protein, low carb ... and lifts weights. (FYI -- his bench is 225)

But hormones helped too -- Beatie says he went back on testosterone 4 months ago after his last pregnancy ... and it gave him a HUGE boost in the muscle department.

As for his transformation surgery -- Beatie says he has no plans to have a full sex-change operation ... as far as he's concerned, he's legally male ... and that's enough.