Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Employee's first name]
First, we would like to say congratulations on the imminent arrival of the newest member of your family. We hope that all goes well, and that you can enjoy your leave time with your baby.
We have received your request for maternity leave and are processing it. You will hear back from the [name of dept that handles requests] within [time period for processing requests]. If you don't hear back, or if there are any changes in your condition, please contact us right away.
In the meantime, if you have any questions are concerns about the maternity-leave process, please don't hesitate to give us a call.