Natural family planning can be extremely effective
if a good method is used and it's followed very carefully.
For information see the "Natural Family Planning Primer"
co-authored by me. I think there's a link
on my website:
http://www.ncf.ca/~an588

For example: the more effective nfp methods
involve waiting until after ovulation each cycle.
Such methods can be made far more effective by
waiting one extra day after the criteria of the
method have been satisfied; if the signs that
ovulation has passed are still there, there's
very high confidence of infertility until the
end of the cycle. Each couple can control the
degree of risk they wish to take.

NFP during breastfeeding can also be highly
effective. The natural infertility of
lactation amenhorrea combined with careful
watching for signs of fertility and following
well-designed rules even if you think you're
probably not fertile can lead to very effective
conception control. At least one study involving
hundreds of women had a zero pregnancy rate.

Since nobody makes much money from NFP, it
tends to be ignored: no advertising budget.