It is best to get your due date on my calendar as soon as possible. My calendar fills up quickly so get your due date to me early.
Newborn sessions are completed within the first ten to fourteen days of life so it is best to get your due date on the calendar during your pregnancy.
For Maternity sessions I usually will leave it up to you when you want to book your maternity session, and when you feel comfortable doing them. I do recommend 34-38 week range.