Here's the deal ... Machine Gun Kelly and Jelly Roll just released the vid for their collab, "Lonely Road," and Megan's got a starring role as a pregnant woman who gives birth by the end of the song.

At the beginning of the video, Megan debuts the baby bump ... and MGK is kissing and caressing her belly in a few scenes. Pretty wild, right?

At the end of the video, MGK and Megan are playing with a baby girl, and the credits say ... "Introducing: Baby Violet Leika."

Some fans online are speculating Megan is legit preggo, and has given birth to the couple's first child ... but that doesn't seem to actually be the case here.

For starters, this would mean they started filming the music video in the thick of a possible pregnancy and then waited a good long while after birth to finish the project.

Doesn't seem likely ... because Megan and MGK haven't announced anything about a pregnancy or baby, and when he posted a photo of them in April on his 34th birthday she had no signs of a bun in the oven.

They've also been at other public events since then -- including Michael Rubin's white party earlier this month -- and there was still no outward sign of pregnancy.

Instead, a woman online is claiming her baby is the little girl who appears at the end of the video ... and the kid is 9 months old.

So, this appears to be a case of some good Hollywood acting and costume work.