。
We're having a baby next month.
Literal
Next-month baby [subject-が] be-born-[explanatory-んです].
赤ん坊 ('baby') is a native-Japanese word; slightly more formal than 赤ちゃん. 生まれる ('to be born') is intransitive — the baby does the being-born. The んです ending frames the statement as a piece of information the speaker is sharing, softer than bare 生まれます.