UFC's most dominant couple -- Amanda Nunes and Nina Ansaroff -- have announced Nina's pregnant and due in Sept.!!!!

"I wanna tell every single person on this planet that Raegan Ann Nunes will be here mid September," Amanda said on IG ... "I cannot wait to see her."

Nina -- the UFC's #5 ranked strawweight fighter -- added, "I am carrying and still plan on fighter after she is born."

The two have been together for years -- and when they came to the TMZ office back in December, they already had big baby plans!

"I’m taking a year off," Ansaroff told us ... but when she returns, Nina says she plans on moving up a weight class and competing in the 125-lb flyweight division.

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

One of the people who reached out to congratulate the couple is Mackenzie Dern, the UFC fighter who returned to action just 4 months after having a baby.

Nunes and Andaroff told us they love the fact Dern got right back to work, but they feel no pressure to get back so quickly.