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

She is expecting a baby / she is pregnant

"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"

Thank you very much! :)

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