PIP: For Catholics who relied on calendar rhythm for their family planning method in the 1950s and 1960s, natural family planning provides an alternative which is more effective, moral, and without the harmful side effects of other forms of contraception. Natural family planning refers to methods of avoiding or achieving pregnancy that cooperate with rather than suppress a couple's fertility through the use of drugs or contraceptive devices. Where the rhythm method simply used mathematical calendar calculations to predict ovulation, natural family planning carefully observes the signs and symptoms of fertile and infertile periods. By monitoring these symptoms and abstaining from intercourse during the women's fertile period, a couple can effectively practice natural family planning. Most studies by the World Health Organization have rated the method 98 to 99% effective. Positive side effects are also experienced by many couples. The challenge of abstinence requires the husband's cooperation and often improves a couple's sexual relationship by providing opportunities for more communication, new and creative ways of showing affection, and mental, physical, and spiritual enrichment.