personally i'd wait for a few more weeks - i tend to find anything taht's been cooked and left in the freezer more than about three months starts to get frost bite and doesn't taste any good (and really isn't good for you)

i just spent this weekend cooking - got a fair bit of cooking done but divided it between myself and a couple of other people and i know a lot will be used before bub arrives. i think if you can get in the habit of doing one thing a week or fortnight between yourself and your partner, you can get a decent turnover of freezer food without putting undue strain on yourself. make sure you date everything you freeze too - then you'll know how long it's been there!

the ONLY way i'd start at 31 weeks is if i intended to use that towards the end of the pregnancy while i'm starting to stock the freezer with more (so you cook now, and then what you'd normally cook fresh at 36+weeks, you freeze, and use the stuff you've frozen now kwim)