Megan Fox Says Daughter With Machine Gun Kelly Wasn't Planned

Megan Fox didn't anticipate her most recent pregnancy ... revealing she and Machine Gun Kelly weren't looking to have a kid together -- even though they're now so happy they did.

The actress took to Instagram to share a steamy video showing her posing in a low-cut crop top, skirt and high black stockings.

She's striking a series of scandalous positions in the clip ... touching her fingertip to her tongue, putting her hands in prayer position and making sultry moves toward the camera.

In the caption, Megan explains that this was taken last year when she was six weeks pregnant -- adding "unplanned but a happy surprise" to the note.

She goes on to write women shouldn't listen to the patriarchy, and they have no expiration date ... before encouraging her fans to watch her new show "Overcompensating."

As you know ... Megan gave birth to her fourth child back in March -- her first with MGK. Her rapper ex announced the news with a black and white photo of their daughter gripping his finger with her tiny hand.

While Megan and MGK haven't been on great terms since they split during their Thanksgiving vacay last year, they were spotted out together earlier this month -- putting aside their drama for their daughter's sake.