The tag alias #54072 "mpreg" -> pregnant_male has been rejected.

Reason: "mpreg" was the 'industry term' for the subject for as long as I can remember, and iirc the tagname used to be mpreg here?
Don't know what happened

EDIT: The tag alias "mpreg" -> pregnant_male (forum #316563) has been rejected by @Rainbow_Dash.

I have no idea why you put "mpreg" in quotation marks.
An actual alias for a working tag can be found in topic #29760 – it's still pending approval, though.

Make sure you run a search for aliases (use the name field) and BURs (use the script field) to see if your request is already in place.

Also, to answer your question, it turns out the male_pregnancy tag was reversed to pregnant_male to match all the other pregnant_* tags. This required removing all the existing aliases first because the BUR system is stupid like that, and then the staff, as is not uncommon, forgot to add them back again afterwards.

The tag alias "mpreg" -> pregnant_male (forum #316563) has been rejected by @Rainbow_Dash.