Generally, you do not use the article when you talk of becoming something unless it is followed by phrase describing the type of thing you will be. I probably made that clear as mud, but examples should help. "I am going to be a mother" = "Voy a ser madre"; "I am goiing to be a good mother" = "Voy a ser una buena madre". The applies to just about anything that one can become. "I am going to be a lawyer" = "Voy a ser abogado"; "I am going to be an honest lawyer" = Voy a ser un abogado honesto."
In spanish we dont use "una" in this kind of sentence. The sentence means that the woman is pregnant and she is going to become a mother, but in general. If you say "Voy a ser una madre" (wich is wrong) it would mean that she's going to be THE mother, someone special or something.
wouldn't that be 'voy a ser LA madre'?? As a rule after SER is not used as its usually descriptive of that person, i think its just different language rules, but occasionally we do the same omissions, like plurals of the same, or when there is only one position like King.
It's a consistent form likewise with professions, I believe, to exclude an article for such a sentence: "Voy a ser poeta". I would venture to say that, were you talking about a more specific mother, e.g. "I'm going to be the mother of a genius", then you would use the article "la": "Voy a ser la madre de un genio." But this is only my speculation, as I'm not a native speaker and am only building upon my limited knowledge. Someone more knowledgeable would have to confirm this.
I don't think the Spanish sentence has the same meaning as the British expression. The idiom "I'll be mother" effectively means "I'm going to act like mother" or "pretend to be mother" and pour you some tea (e.g. children playing at a tea party and using "mother" as a proper name rather than a general noun), whereas the Spanish sentence means literally that I (a female person) am going to become a mother, i.e. give birth to my first child.
I'm a native Spanish speaker. :) You're actually right when you write: "I'm going to be mother" (I think. And I'm saying 'I think' because I'm supposing that you are in the context of a pregnant women). Technically, we Spanish speakers would translate your sentence as: "Voy a ser madre", wich means literally "I'm gonna become a mother".