I decided to google commentaries on Genesis 3:16 and most of them didn't deal with this specific issue, until I stumbled across this article from "gracethrufaith". For me, it settled the issue in my heart that nowadays, post fall, women conceive more frequently than they would have done pre-fall. Women were not originally designed to have 20+ babies, this is a result of our fallen nature.
That said, I'm not looking down on couples who have decided to go contraception free and have large families. The Duggar family has a really good website outlining their reasons for doing so and I bless them for it, I don't think they're doing anything wrong or sinful. They are raising their children in the way of the Lord- that is admirable and God will bless them for it. I'm also not standing in judgement over couples who decide to contracept in order to devote more time and energy to fewer children.
I do think it's important to know how different methods of contraception work in order to make an informed choice. E.g. until last year I hadn't realised that hormonal contraception (like the Pill and IUD) can work abortively (they do not allow a fertilised egg, a foetus, to implant). I know of one couple who use the Pill, knowing it's abortive properties, but they get around this by using a barrier method (condom or diaphragm) when they know the wife is ovulating (which is easier to calculate and measure when the menstrual cycle is regulated by the Pill.
My own mind isn't made up on family planning at the moment, except that I don't think I would be able for 6 kids never mind 20! As a single woman it's not an issue for me now but it's good to at least have an idea about it before I do get married :)
Handmaiden of Adonai