This is a really common question, and one that needs to be asked, so no bother for me! I love answering questions about NFP. :)
First, let me rephrase the question like this:
Does artificial birth control really work? A lot of the families that I know practice it have a lot of children.
Obviously that sounds weird. A woman may have used the Pill to space her kids, or used condoms in between pregnancies until she felt ready for another. Even if one of the children was a “surprise baby” due to bc or user failure, it doesn’t take away from the fact that the woman spaced pregnancies using her chosen method of birth control.
With NFP, it is no different. When the rules are followed conservatively, it’s effectiveness is on par with hormonal methods. And I would know: my cycles have been odd since they came back after postpartum bleeding, but I just started my sixth cycle a few days ago! Totally not pregnant here, lol. If someone like me can learn NFP on her own using proper literature while breastfeeding postpartum, then a couple can most certainly use it effectively with a trained instructor to teach and aid them.
However, couples who use NFP do tend to have more children, as you have observed. If it is so effective, why do they have children? Shouldn’t they have the two or less numbers like most families?
You have to consider this: most people who use NFP are devoutly religious, and believe that they should only avoid pregnancy in an emergency, such as emotional distress or financial problems.They do not follow the small family trend nor should they be expected to.
While each couple will have to discern on their own whether they are ready for another baby, either they use a method of NFP to space their children by a couple years or they simply do not use NFP at all, letting babies come as they may and only applying NFP when the need arises.
Not every family desires so few children, and many who use NFP love large families! Because barrier methods are not as effective in avoiding pregnancy and hormonal bc can take a while to wear off when no longer being used, it makes sense that those who want lots of kids will want a natural method that does not hinder their fertility and will aid them when pregnancy is desired.
Speaking of desired pregnancy, another reason is that NFP is more than just a form of birth control: it is also used for pregnancy achievement. Couples struggling with infertility will not ask for a prescription of hormonal bc, or an insertion of an IUD. Instead, they may use charting methods of NFP to increase their chances of conception. So when someone says they use NFP, it could be for birth control, pregnancy achievement, or a combination of both depending on what their life situation is like.
Finally, this general openness for children often leads to my next point: if a couple does not have a serious reason to avoid pregnancy, they may be lenient with the rules! Because the thought of another baby isn’t a death sentence, they would take more risks and not care too much if it ended up as a pregnancy. As mentioned before, NFP is effective but only when the rules are applied conservatively. Otherwise, loosely following such rules will increase chances of pregnancy.
So to answer your question: yes, NFP is effective! It simply depends what “effective” means for each couple, as it could be used for pregnancy achievement or pregnancy avoidance, or they simply chart while taking chances because they neither dread nor desire another pregnancy.
I hope that answered your question! It’s a legitimate concern people have and one that deserves answering. If you want to learn more, feel free to check out iusenfp.com. It has a lot of sources and also an FAQ that could possibly answer other questions you may have.
Related ask: http://patron-saint-of-smart-asses.tumblr.com/post/92080035054/did-you-see-the-new-post-in-the-nfp-tag-i-was