A maternity letter is used to inform your employer about the time off you'll be taking for the birth of your child.
If the letter is being sent as a hard copy, then it should be in the proper business format, otherwise, if the letter is being emailed, it should have an appropriate subject line and proper grammar.
I am pregnant and due on or around [Expected Due Date]. This is formal notice of my upcoming maternity leave.
I understand that our company's policies allow six weeks of paid leave. I do plan to take 12 weeks, with six weeks of unpaid time off.
I plan to work until my due date, unless my doctor advises me to leave full time employment earlier for medical reasons. This would give the parameters for my leave as approximately [Month/Day/Year - Month/Day/Year].
Before I go on leave, I will do all I can to make sure that my projects are completed before my leave. I also welcome the opportunity to train my replacement. Further, during my maternity leave, you may call me at (555) 555-555 for any emergency questions that arise about my usual job duties that cannot be resolved except by speaking with me.
If there are any changes, I will update you as soon as I am able. Thank you for your understanding and accomodation.