Does such a word exist?
It has occurred to me that the "woman" part is redundant, since only women can be pregnant (except for Arnold Schwarzanegger in Junior)
Primagravida is a medical term for somebody pregnant for the first time, and multigravida is in the OED for somebody pregnant not for the first time, so you could coin gravida, I suppose. But in normal usage, I'd say there is no such noun.
Extremely informally, people may say a "preggo"
If you can accept a hyphenated phrase, then mum-to-be or mother-to-be are quite common.
(Or mom-to-be in the USA, I presume?)
The OED records pregnant as a noun as well as an adjective, with the meaning ‘a pregnant woman’, with three citations from the twentieth century alone. I wouldn’t say it was in common use though.
The real frain is whether such a word is needed.
OE had byrþestre (e) f. female carrier. So I guess a modern version could be: birthster.
A bold use of the word Alpha-bitch or even Queen might suffice.
Mums-to-be could also be known as 'Ovens' or 'Hosts' (depending on whether the baby is referred to as a bun or a parasite).
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