Someone on Facebook posed the question: "Why do all NFP teachers seem to have 5 kids?" It's a good question: if NFP works, why are these people always pregnant? One answer is that they want to have lots of children, which is a good thing but does raise the question of whether they practice the system they preach. But probably a more accurate answer is: do you know how many more kids we'd have without NFP?
So I did a little math on the subject I know best: myself. I have been married for 14 years minus 5 months, so 163 months. I had my first baby 15 months after I was married. My fertility returns, on average, 6 months postpartum (second child, 15 months younger than the first; and careful observation after the other four births bears that out). So, a fifteen month cycle between possible pregnancies. Had I not used NFP and gotten pregnant on a fifteen-month schedule, well, 15x11= 165. After being married for 162 months, I could be six months pregnant with my 11th child.
Let me restate that. I could have 11 children by my 14th anniversary without NFP. I will have six, almost half that number.
Now this isn't carved in stone. One won't necessarily get pregnant in a given cycle; I myself have gone several cycles without conceiving even when I was trying to. (Observation says that results in boys, BTW.) But you can't know that except in hindsight, and I do know that I can get pregnant even when I'm trying not to conceive; it's happened three times. (Observation: A girl, a miscarriage, and a girl.)
After the surprise of my second pregnancy, all my children have been spaced with NFP. The second closest spacing is 22 months -- still closer than I'd intended, but far saner and healthier than a 15-month spacing. As I get older and my pregnancies get tricksier, that spacing -- and the physical condition it allows me to recover -- become more vital. I was only 23 once.
So, I'm not an NFP instructor, but people may see my six kids and wonder, "Does NFP work?" And the answer is, "Do you see my 11 kids? No? Then I can assure you that it does."