In Mexico people calls mamacita to his/her mother, sometimes to girlfiends, young girls in close family, as an affectionate word, but is used also to hot or sexi woman. (Adios mamacita/ es una mamacita)
I think, if you are close friends, you can adrdress her "mamacita" and explain why.

Perhaps you want to say her "futura mamá" in a formal way.

Thanks for the idea. Futura Mama. I like that better.

Yes, I would go with vernic's suggestion "futura mamá",

Mamacita can mean a sexy woman ( not in Spain!!) but as it also means mom...I don't' think you were misunderstood.

I know you probably don't have accents in your computer (you do, actually, but it is not obvious), but take care at least when you pronounce it, because "mama" means "breast"; "mamá" means "mommy".

"mama" can't be mommy?

So do I call my mommy "breast" without knowing?

Estoy alucinando

Mama is also a verb form that means nurse (suckle) and, in a cruder sense, something else...

And I don't understand why Dunia is questioning your advice. Is there any part of the Spanish-speaking world where this does not apply'

cáncer de mama means breast cancer not mummy's cancer

"Mama" has two meanings in Spain: breast and mummy.
You can also say "mamá" but for me it is more common "mama".
When I speak with my sibling about my mother we always say "la mama".
For example: "La mama me ha dicho..."

I'm not saying that "mama" as mummy aplies in any context, it is used usually in home environment.

For example you can't say "futura mama".

It depends on who you are talking to and about what you are talking about.

Thank you. I suppose it's a personal choice. I call my mother mamá

As a recently pregnant woman (last year), here's my opinion for what it's worth. Although I'm not a native Spanish speaker, I am aware of the Mexican slang meaning of "mamacita" and would definitely NOT use it with my pastor's wife. (Although I'm sure she knew that wasn't how you meant it.) You could use "mamá" or "mamá nueva", but I wouldn't do it all the time. It gets annoying after a while. After all, I assume she has a name.

A friend of mine actually liked the double meaning, implying that although she was to become a mum, she was still also seen as a sexy mama', or hot woman!