Natural Family Planning is as effective as it is because the couple does not have intercourse during the fertile days.
Opting to use barrier methods of contraception, such as condoms, diaphragms, spermicides or withdrawal during the fertile time, reduces the effectiveness rate down to the effectiveness of the particular barrier method used. Using such methods is not Natural Family Planning. Pregnancies may occur. There is no method more effective than abstinence during the fertile time.