Your cycle can vary in length
“A normal cycle lasts anywhere between 24 and 38 days, most commonly 28 to 35 days,” Dr. Brodman says. “Some women have the same cycle interval like a clock, but others find their cycle interval is less predictable.”
Because the luteal phase is always 14 days, changes in the length of the follicular phase are what change the length of your whole cycle. “A short cycle has a short follicular phase and a long cycle has a long follicular phase,” Dr. Brodman says. And because the length of your follicular phase changes, that means ovulation isn’t always as predictable.
“If you have a short cycle, you may actually be ovulating on day seven or eight of your cycle. And if your period bleeding lasts a long time—say, seven or eight days—then you might conceive even though you’re technically still on your period,” Dr. Brodman says. Plus, “even if you always have predictable periods, from time to time you may have an early or a late ovulation.” That’s why using the “rhythm method” as contraception doesn’t always work. And you won’t really know, since you’ll just have your normal period.