NFP 2

How effective are the methods of natural family
planning for avoiding pregnancy?

When couples are taught by competent teachers, understand
the methods, and are motivated to follow them, NFP is up to
99% successful in spacing or limiting births.

The effectiveness of NFP depends upon the spouses' following
the rules of the method according to their family planning
intention (i.e., achieving or limiting births). Those who are
strongly motivated to avoid pregnancy and follow the method
strictly are very effective in meeting their goal. (Reference -
The Diocesan Development Program for NFP, National Conference of
Catholic Bishops)

For a more detailed answer to the question (regarding each of
the different methods) visit the article "What are the different
methods of natural family planning?"