Honestly, I don't know! Maybe part of the issue is how mpreg differs per author. It's so subjective. There is no "right" way to write or think about mpreg, so everyone comes at it from different perspectives, and it would seem most of them don't include menstruation.

You know, I've noticed a lot of us mpreggers have glossed over the fun (?) of menstruation (including myself)! This will not do.

And take birth control and spend money on tampons and pads and worry if the pad is making your pants look weird etc.

Yeah! I guess there are other people just discovering this stuff, but it was so commonplace in all the fandoms I was a part of as a teen and young teenager.

It's natural to be curious about biological experiences you cannot have. We don't talk about "penis envy" much anymore in psychology (because LOL), but that was a thing women were suspected of having for a long time. I think it's normal for most cisfemales to wonder what it would be like to actually have a penis at…

Have at it, boys. I have zero objections to sharing the burden of replicating the species. But fair is fair. You also have to copiously bleed from you penis every 28 days with the side effect of extremely painful cramps and you have to keep this secret because of social stigma. Have fun!