[jwcacces]

Because it's hard work, and it states that both partners are going to do that work.

It can be hard work to get pregnant, hard work to stay pregnant, hard work to carry to term, hard work to deliver and it is unbelievably hard work to raise a child. Two partners trying to get to the finish line have to make incredible sacrifices, and make huge changes to their lives in order to support each other.

Saying we're pregnant and sharing the state of pregnancy vs just saying she's pregnant, acknowledges the the shared work and the shared responsibility that is taken up by both partners, and it acknowledges the shared joy in bringing life into the world. And when you say it to someone other then your pregnant partner, you make it clear to them that you are picking up a huge responsibility and that they should recognize it.