A rope and a pulley work to lift or hoist objects. Wagons roll because they have wheels and axles. These are simple machines. The body, although it has mechanical capacity, is not an "it" but a "who." What works for a machine can be honed down to the function of its parts. What works for a person has way more to do with how a person responds to a process. The answer to the question, is, "Yes, NFP works." It works to help a couple conceive a child and to help them space or limit their family size. It does not however "work" for the same reasons that something mechanical works. 
NFP is a process of discovery. It is not blocking, altering, poisoning, or aggresively eliminating the natural order of things. It works because it is healthy for our bodies and our spirits and respects that delicate balance that we are. God designed our bodies with mystery and purpose. With NFP we are able to discover the unique and personal design of our female bodies and share that information with our spouse.
Many of us that use NFP tend to have larger families, not because the method has failed us, but because we have an openess to children that others may not. We have accepted the formula of union=babies, and have decided that we will accept children from God as the miracle that they are.
We all want successful marriages. If I were to pick one of the major things that has "worked" for our marriage it would be NFP. It lifts like a pulley; it rolls like a wagon, but it "works" in ways that go far beyond mechanics. The image is more like a tree. It functions to give oxygen and shade, and yet it is beautiful and grows all at the same time.
Do you want more for your marriage? Do you want a way to plan your family in a way that goes beyond functioning? Aim for more. You are more than a machine.
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