At around 8 months is probably the best time if you are a first time mum and probably slightly earlier when you are having your second or third baby.
At this time your bump will be nice and round and you will still feel comfortable enough to move around and do a few poses :-)
It is also a good idea to think about what type of photos you would want out of your maternity shoot. Would you go for the more artistic approach where photos are mostly black and white with minimal lighting or would you rather go for the more light, natural look with pretty maternity clothing and nice backgrounds and props or perhaps an outdoor photo shoot?
All of these things are worth bearing in mind before having your maternity photography session.