Actually we call them mothers-to-be, or more often, pregnant women. I had no idea you called pregnant women "Mothers". Mirriam Websters defines Mother as:-
A woman in relation to a child or children to whom she has given birth.
Bring up (a child) with care and affection: "the art of mothering".
Perhaps it has become more colloquially used over here in the USA. Do you also call the man who inseminated her a "Father"? I have often heard older women announce "I am going to be a grandmother", not "I'm a grandmother", before the birth.
Actually UnOne, for once, I think you have an excellent idea - "fetus-carriers" - I like that. I wonder how many young mothers-to-be, sorry, fetus-carriers, are going to like it as well?
Might be a good way to separate the "cake havers" from the "cake eaters" --
Few, I'm pretty sure.