Why Are Period and Fertile-Window Predictions So Often Wrong?

Calendar predictions assume ovulation happens on a fixed day of your cycle, usually day 14. It doesn’t. Ovulation day varies widely between people and from one cycle to the next in the same person, which is why studies find that apps and calendar methods predict it no better than about one time in five, and why a predicted “safe” window can’t be relied on to avoid pregnancy.

This article is general information, not medical advice. For contraception, talk to a health professional about effective methods.

What the Studies Found

• Calendar apps. Johnson and colleagues measured the hormone surge that triggers ovulation in 949 women over a cycle, then compared it with the predictions of cycle-tracking apps and calendar methods. The apps’ accuracy in predicting the day of ovulation was no better than 21%. Even for women with 28-day cycles, the most likely ovulation day was day 16, not day 14, and only on 21% of cycles.
• The fertile window moves. Wilcox and colleagues followed 221 women planning a pregnancy, timing ovulation from hormones in daily urine samples. On every day from day 6 to day 21 of the cycle, women had at least a 10% chance of being in their fertile window, and in only about 30% of women was the fertile window entirely within days 10 to 17, the days clinical guidelines used to give. Some ovulated much later, and a late ovulation can’t be predicted.
• Most variation is in the first half. Bull and colleagues’ analysis of 612,613 cycles found that the time from a period to ovulation varied far more than the time from ovulation to the next period, so a cycle’s length isn’t known until it’s over.

What This Means

• Predicted periods are rough guesses for most people, especially with cycles that vary by more than a few days.
• Predicted fertile windows aren’t contraception. Fertility awareness methods that work track signs of ovulation in the current cycle, such as basal body temperature and cervical mucus, under specific rules, and even then their effectiveness depends heavily on how they’re used.
• What’s reliable is the past. Your cycle lengths, period lengths, and where you are in today’s cycle are facts you can use, and they’re what a doctor will ask about.

Following It in Kaizora

Kaizora makes no predictions about your next period or a fertile window, for these reasons. Menstrual Cycle says where you are as facts: the day of your current cycle and the range of your recent cycle lengths. It charts your cycles aligned by cycle day, with flow, test results, and temperature on each, so you can see your own pattern, including when your temperature rose. See What’s a Normal Menstrual Cycle and Period Length? and What Can Basal Body Temperature Tell You About Your Cycle?

The Short Version

• Ovulation day varies widely, even in regular cycles.
• Apps predicted ovulation day correctly no more than about 21% of the time in one study.
• A predicted fertile window isn’t contraception.
• Your recorded cycles are more reliable than any prediction.

Sources

• Johnson S, Marriott L, Zinaman M, “Can apps and calendar methods predict ovulation with accuracy?” Current Medical Research and Opinion 34, no. 9 (2018): 1587–1594. doi:10.1080/03007995.2018.1475348
• Wilcox AJ, Dunson D, Baird DD, “The timing of the ‘fertile window’ in the menstrual cycle: day specific estimates from a prospective study,” BMJ 321, no. 7271 (2000): 1259–1262. doi:10.1136/bmj.321.7271.1259
• Bull JR et al., “Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles,” npj Digital Medicine 2 (2019): 83. doi:10.1038/s41746-019-0152-7