Well really we want the biggest bump possible! So the response is as late as we can without running the chance of babe arriving before the shoot!
Around 4 weeks before your due date is ideal. But that is solely a guide, anywhere from 12 to 2 weeks before your due date will be fine.
If you are having maternity leave, then you can certainly wait until stop working. This gives you a chance to be more relaxed and enjoy the shoot instead of feeling like it has to be fitted in.
You may want to do a "gender reveal" - where you use the photographs to announce the sex of your baby. You probably won't discover this until your 20 week ultrasound. You will have a delightful bump by then if you want to get the news out as soon as you can.