What is the difference between the Rhythm Method and modern NFP?
Calendar Rhythm basically assumed that women would have 28-day cycles with ovulation around Day 14. It was ineffective if cycles were either shorter or longer. Modern NFP acknowledges that a woman can routinely have cycles that are shorter or longer than the “average” of 28 days, and that her cycle can even vary from month to month. NFP users make their interpretation choices based on the fertility symptoms they are currently experiencing this month, which is why it is still highly effective for women with irregular cycles, unlike Rhythm.
