It's kind of true for me. My son (who was my first) was unplanned and was conceived the first month my ex and I were together. My second child was also unplanned (messed up birth control) and she was conceived that month. So I got pregnant the first month both times. However, with my third, it took me 13 months to conceive her (we were NTNP).
This time around, I'm on month #11. I've gotten pregnant twice (after 7 months TTC, and again after 9 months), but it's definitely taken longer than my first two children. And it's looking like it'll take as long as, if not longer than, with my third child.