Kristin Cavallari & Jay Cutler -- Pregnancy Was an ACCIDENT!!!

If you were surprised to hear Kristin Cavallari was pregnant with Jay Cutler's baby, you're not the only one -- the couple themselves were shocked by the news ... TMZ has learned.

Jay and Kristin announced yesterday they were with child -- following a crazy few months where they broke off their engagement, only to announce it was back on.

Sources close to the couple tell TMZ, K&Jay had discussed the possibility of kids -- but planned to start cranking 'em out in a few years ... not any time soon.

But shocking as it was, we're told they're thrilled with the news ... renewing their commitment more than ever to making their relationship work.

If Jay wasn't locked down before, he sure is now.