We did the math (hey, numbers are hard) ... and it's only been 124 days since 26-year-old Mackenzie became a mom. She gave birth to her daughter, Moa, on June 9.

Think about that ... not only did Mack have to recover from the toll that pregnancy and delivery takes on your body, but then she began to train for a FISTFIGHT with a world class opponent almost immediately.

Dern -- who's 7-0 as a pro MMA fighter -- just hit the scales for the official weigh in for UFC Fight Night 161 in Tampa, Florida ... and came in at 115 lbs, a perfect number for her weight class.

Mackenzie -- who's known as one of the top female Brazilian Jiu Jitsu practitioners -- is set to face Amanda Ribas, who did NOT give birth 4 months ago!

And, get this, according to Vegas odds ... Dern is still the slight betting favorite ... you have to put down $145 to win $100.

FYI, Mackenzie's baby daddy is Brazilian pro surfer Wesley Santos ... so that kid is gonna be freakishly athletic!!!