Why do fertility awareness methods work better with regular menstrual cycles?

Fertility awareness methods, also known as natural family planning, rely on identifying the approximately six-day fertile window during a woman's monthly cycle to avoid or achieve pregnancy. These methods work more effectively with regular menstrual cycles because a predictable cycle allows a woman to more accurately estimate when ovulation will occur.123

In women with irregular cycles, the date of ovulation is uncertain, often because the proliferative phase of the cycle varies in length. Because these techniques—such as tracking a calendar, monitoring basal body temperature, or observing cervical mucus—are used to predict the fertile phase, a consistent cycle provides a more reliable framework for these observations. Regardless of regularity, however, the effectiveness of these methods depends heavily on a high level of motivation, consistent record-keeping, and proper training.312