The pregnant "Transformers" star proudly displays her bun in the oven in a series of shots posted on social media ... featuring Megan wearing some black lace lingerie.

As you can see, Megan is cradling her baby bump in a sheer look ... covering up her private parts but showing lots of skin.

Megan's also got long, red nails ... and there's video of her posing at the photo shoot too.

The social media posts make it seem like this was Megan's look for last night's GQ Men of the Year 2024 party at the Chateau Marmont in Los Angeles ... as we reported, she pulled up to the party with MGK but never got out of the car.

As a result, this is the first time we're getting a good look at Megan's baby bump.

Megan announced earlier this week she was expecting her first child with MGK ... and, as we first told you, our sources say she's 4 to 5 months along in her pregnancy.