Nina is the 5th ranked UFC strawweight contender in a STACKED division that includes killers like Joanna Jedrzejczyk, Rose Namajunas and the champ Weili Zhang!

And since Nina plans on continuing her MMA career after she gives birth, she planned on staying in shape during the pregnancy to help expedite her return to the Octagon.

Take a look at the video ... Nina and her fetus are WRECKING those pads!

Of course, Nina is engaged to UFC champ Amanda Nunes -- and when they came into the TMZ office before the COVID pandemic, they spelled out their fight/baby plans!

"I’m gonna be training throughout my pregnancy," Nina told us ... "I’m gonna stay in shape. I’m not gonna, ya know, just like not do anything and jump in."

Nina said the plan is to take a year off -- but acknowledged, "I’ve never had a baby before. So, I don’t know what to expect."

As for Amanda, she obviously supports Nina -- but doesn't want her to rush back to the Octagon too soon ... a mistake she feels Mackenzie Dern made earlier this year.