Best time to take maternity photos is generally during first part of your third trimester. So anytime from 28 to 35 weeks. Don't wait up though, those babies are unpredictable ;)
Pregnancy sessions are one of my favorite to shoot - I will gently direct you and all the people you love most that are with you on how to pose without feeling awkward and stressed. I regret to this day not having my maternity photos done professionally... just saying.