Frequency: A pregnant woman, especially one expecting her first child. noun.

What is the meaning of mommy to be?

Word forms: plural mothers-to-be. countable noun. A mother-to-be is a woman who is pregnant, especially for the first time.

Is it correct to say mommy?

Both spellings are correct and derive from mommy and mummy. According to Ngram, Mom/mom is the more common spelling in AmE, while Mum/mum is more common in BrE: Mom : 1867, American English, perhaps a shortening of mommy; also see mamma.