When cycles vary, a range beats a date

Because an average hides the thing that makes a cycle irregular. If your cycles run between 26 and 38 days, the average of 32 is a day your period has possibly never arrived on. The range covers where it actually lands.

Using the calendar method: the first fertile day is your shortest cycle minus 18, and the last is your longest cycle minus 11. The 18 accounts for sperm surviving up to five days before a possible early ovulation, and the 11 for a possible late one.

At least six gives a usable shortest and longest. With fewer, the range you enter probably understates how much your cycles move, and the prediction will look more confident than it should.

Cycles shorter than 21 days or longer than 35, or a difference of more than about 9 days between your shortest and longest, are generally treated as irregular. Occasional variation is normal, especially in the years after a first period and during perimenopause.

Common causes include PCOS, thyroid conditions, significant weight change, intense exercise, chronic stress, perimenopause, and some medications and contraceptives. A calculator cannot tell them apart, and a persistent change deserves a clinical answer rather than a better prediction.