To write a letter to an employer requesting maternity leave, state clearly when the maternity leave begins and the projected date it ends. If you are not sure when you can return to work, state so in the letter. Be as clear and as truthful as possible in the letter.
Address your letter formally, and start it like any other business correspondence letter. How detailed you make the letter depends largely on your relationship with your employer, and always err on the side of clarity to provide necessary details. There is no need to share intimate details regarding the pregnancy.