I am a little surprised that they are delaying this ultrasound. However it is important to remember that very late ultrasounds are notoriously inaccurate for predicting final birth weight. I would be more thinking about checking fluid levels, as if you go into labour before you have the scan and you have a lot of fluid then that is the sort of thing the midwives will want to know and keep an eye on.

How big was your first baby, and did you have any difficulties having a vaginal birth? Have you any specific concerns about having a big baby? It could be that you just cook them large, and most often this is not going to cause a problem at birth as many tiny women manage to birth very large babies without complication.