How should I say: "she will have a child" or "she will has a child" and why?

Question

Answer

Hello Jakub,

'Will' is part of the family of modal verbs and they are always followed by a bare infinitive (1st form - no 'to'). So 'she will have a baby'. Other members of this family include 'should', 'would', 'must' ...

You can also use both phrases given by Valery, both natural.

Hope this helps

Bob

Answer

She is expecting a baby / she is pregnant

Answer

"She will have a child" is correct.

I'm not an English teacher, but I believe the reason why is the future tense is expressed as "will" + [infinitive]
Examples: "he will walk" instead of "he will walks"
"she will give" instead of "she will gives"

Answer

Thank you very much! :)

Answer

She will have a child. 
After WILL and WON'T, use the base form (bare infinitive) of the verb.