Amanda Bynes says she's got a baby on board ... and she wants the world to get the first glimpse of her upcoming bundle of joy.

Bynes stunned fans Tuesday when she posted a pic of an ultrasound with the caption, "Baby on board!" Amanda didn't specify, but the daddy-to-be is likely her boyfriend Paul Michael who she's also recently introduced the world to.

The ultrasound appears to be legitimate, with Amanda's name on it and Tuesday's date. Unclear how far into her pregnancy she currently is.

As for her relationship with Paul, it has been rocky as of late ... the two announced they were engaged shortly after Bynes posted about their relationship. The engagement was quickly called off ... but as of late, it appears the two have been back together.

What's not clear is what Amanda's family thinks of the announcement ... Bynes is currently under a conservatorship and family sources tell us they would not have allowed Amanda to get married. We learned her engagement ring was a phony too, a $49 purchase on Amazon.