What am I supposed to say?

Hubby and I have been infertile for at least half our marriage (10+ years married). It might have been longer infertile but idk because we used condoms for several years. People have asked when we gonna have kids and secretly poopooed our lack of children behind our backs. I've always brushed off the question with something vague about being good with not having kids and not mentioning the infertility (it's personal and not something I freely share). Well by some crazy chance, I now find myself 13 weeks pregnant! We have told most of our family but one question I've got a lot is "what is it on purpose?" Which personally I think is an invasive question but whatever. I can't exactly say "yeah we've had loads of sex without birth control but suffer from infertility. This little booger actually got through! Must have been a great sperm and egg!" Or "idk we've been raw dogging all these years, so weird that it worked this time." So what is a nice way to address this question?