Statistically NFP is as effective as any contraceptive and true surprise pregnancies (those that occur despite consistent, correct application of the rules to avoid pregnancy) are so rare that they can honestly be considered little miracles, but they do occur and here we come to the heart of the difference between NFP and contraception - the question of who is really in charge of planning our families.
The answer is that it is always and only God who is charge and that He has a specific plan for each of our families that only He knows the whole of. He reveals this plan to us in bits and pieces as it is time for us to act to cooperate with it, but usually He doesn't let us know years in advance exactly how many children He has in mind for us, nor how they will be spaced. What He asks of each couple is to prayerfully and honestly try to discern His will for them and then to cooperate with that plan.
A couple who believes that it is not God’s will for them to have another child at present cooperate with God and do what is right by using NFP to avoid pregnancy. If a couple conceives despite their best efforts not to, however, they can then trust that God is telling them, in no uncertain terms, that His will for them has changed. It doesn't mean that they necessarily did anything wrong in their use of NFP or even that they incorrectly discerned God's will when they determined that they ought to avoid pregnancy. It simply means that right then God is asking them to accept one more child . . . right then.
This can be shocking and upsetting because we are used to thinking that we are the ones in control of our lives. But we aren't really. Ever. In any area of our lives.
When scientific literature discusses the ‘failure rate’ of a method of family planning this refers to the number of pregnancies that occur despite correct use of the method and even in this sense NFP is no more likely to ‘fail’ than other methods of family planning. Really, however, there are no ‘failures’ with NFP because even children whose conception completely surprises their parents (and their parents’ NFP teachers) are specifically willed by God. A couple who uses NFP is far more likely to understand this than a couple who contracepts because the cycle-to-cycle discernment process that they must go through in order to continue to avoid pregnancy encourages them to recognize that they are to be cooperating with God’s plan for their family rather than doing the planning themselves.
God sees the whole picture and we do not. He loves each of us and each of our children more than any of us can possibly imagine and we must learn to trust Him. And this is what Catholic family planning really amounts to for all of us – trusting that God is in control, even as we work to cooperate with His plan for each of us, and that “in everything God works for good with those who love Him” (Rom. 8:28).
This is what faith means. It means to trust in God in spite of the circumstances.