Pre book either session from 20 week scan. If you are due at Easter or September the earlier the better. They are very busy times of year for babies. So if you are booking date for maternity, its best to get you in before you fill to full, so I suggest 30 weeks onwards or when your bump is big but you are feeling glorious. I want you to enjoy and celebrate your bump, personally I struggled with mobility the end of my 3rd trimester. Your newborn session is booked at the 20 week scan but is your flexible slot is based around your due date.Its flexible for obvious reasons, babies tend to arrive off schedule.