Stranger danger!

thats kinda concerning, you should probably get to the bottom of this

I would have asked if she's expecting a boy or a girl but we do have pregnant women close to us. If she hasn't met any then I'd be wondering where she learned that.

you need to figure out who taught her that. it may have been a one time joke that was intended to be harmless, but there could also be something much worse happening with her. now might also be a good time for a light birds and bees talk and explain if anyone touches her there or asks her to keep secrets she comes to mommy right away

Yea I would try to prod how she learned that, don't react too strongly or it might scare her (although she seems pretty confident) and she might not communicate. As gross as it is the youngest mother (not "mother" but actual mother, as in got pregnant, and had the baby via C-section) was 5 years and 7 months when the baby was born. It might be a bad idea to tell her that only grown-ups can get pregnant or she could be like 13 thinking she can't get pregnant since she's not an adult.

That’s hella weird and concerning, like the other comments I’d look into asking gently where she heard that from and make sure it’s not something serious.