My guess is luck. In my little circle of friends, nobody had any trouble conceiving. Heck, some were provably one-and-done (sad though it may sound, one of my friends only had sex with his wife when she was ready for them to have kids. Do it, wait a few weeks, try again, but they didn't have to try twice for either of their kids). One of my friends had two kids and his wife really wanted a third, then they had twins. Ha, whoops!