We started at 29w simply because that's when the classes were best scheduled. The course before ours was too soon (it needs to be reasonably close to D-day so that the learning is not forgotten by the time it's most needed) and the course after was too late (not completed by D-day). The other girls in our course range from ~25w to ~32w. If you are doing private (one-on-one) classes then you can choose your own schedule, but if you are joining with others then you'll need to check their schedules.