I noticed a rash of couples recently who will announce to friends and family, "We're pregnant!" At first I thought this was just a cutesy way of announcing a pregnancy that was intended to be humorous, with the joke being that one partner was somewhat less pregnant than the other. But since first noticing it a few years ago, I've heard and seen it in print much more often.
Here's an example of an image that appears to use it in a humorous context:

Contrast with this image, which seems to be entirely serious:

Is this an acceptable use of the word 'pregnant'? And if so, how far back does the usage go?