Expressing before birth? 38 weeks ... advice please!

Hey! I’m 38+1 weeks pregnant and I want to express rather than breast feed as I have 2 other boys and dad who also want to help do the feeds my midwife said I could start using my pump from 38 weeks and store the milk, has anyone ever done this? How long did you pump for and does it have to be frozen right away or can you fill it before freezing the bottle? My other boys were formulae fed so this is new to me! Got a sweep at 38+4 so don’t want him to be hungry when he arrives :) thank you :)