WordNet records no synonym and no antonym for natural family planning. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
any of several methods of family planning that do not involve sterilization or contraceptive devices or drugs; coitus is avoided during the fertile time of a woman's menstrual cycle
Synonyms, antonyms and the broader and narrower relations for natural family planning come from Princeton University’s WordNet. Full sources and licences.