13%
Of 612,613 cycles logged by 124,648 users of a fertility app, only 13% were exactly 28 days long (Bull et al., 2019).
Cycle and pregnancy statistics
Findings from studies of the menstrual cycle, the fertile window, the calendar method and the due date. Each number gives the study, the year, the sample size and a link to the paper.
Updated October 12, 2026. Every number was checked against the original paper, linked next to it.
01 · The cycle
The cycle
91%
Of those 612,613 cycles, 91% lasted between 21 and 35 days (Bull et al., 2019).
29.3days
In 612,613 cycles logged in a fertility app, the average length was 29.3 days (Bull et al., 2019).
16.9days
In 612,613 cycles, the time from the start of the period to ovulation averaged 16.9 days, and from ovulation to the next period, 12.4 days (Bull et al., 2019).
0.18days
Between the ages of 25 and 45, the average cycle got 0.18 days shorter with each year of age, in 612,613 cycles (Bull et al., 2019).
02 · The fertile window
The fertile window
6days
In 625 cycles of 221 women trying to conceive, pregnancy happened only after sex in the six days ending on the day of ovulation (Wilcox, Weinberg and Baird, 1995).
33%
In 625 cycles of 221 women, the chance of conception from sex on a single day ranged from 10%, five days before ovulation, to 33%, on the day of ovulation (Wilcox, Weinberg and Baird, 1995).
30%
In a study of 221 women and 696 cycles, only about 30% of the women had their whole fertile window between cycle days 10 and 17 (Wilcox, Dunson and Baird, 2000).
10%
In 696 cycles of 221 women, on every day from cycle day 6 to day 21 the chance of being in the fertile window was at least 10% (Wilcox, Dunson and Baird, 2000).
03 · The calendar method
The calendar method
21%
Using data from 949 women who collected urine samples for a whole cycle to find their ovulation, cycle apps predicted the day of ovulation correctly no more than 21% of the time (Johnson, Marriott and Zinaman, 2018).
15%
Of 949 women in a 2018 study, 34% believed they had a 28-day cycle, and only 15% did (Johnson, Marriott and Zinaman, 2018).
04 · The due date
The due date
280days
The due date is set 280 days, or 40 weeks, after the first day of the last period (ACOG, 2017).
5%
Of 18,708 spontaneous births in Australia, only 5% happened on the estimated due date (Khambalia et al., 2013).
International Journal of Gynecology & ObstetricsLink to this number
66%
About 66% of 18,708 spontaneous births in Australia happened within 7 days of the estimated due date (Khambalia et al., 2013).
International Journal of Gynecology & ObstetricsLink to this number
37days
In 125 pregnancies dated by the measured day of ovulation, the length of pregnancy still varied by 37 days, after excluding preterm births and pregnancies with medical conditions (Jukic et al., 2013).
268days
In 125 pregnancies dated by the measured day of ovulation, the median time from ovulation to birth was 268 days (Jukic et al., 2013).
The numbers in the calculators
Sources
- Bull JR, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine, 2019;2:83.
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine, 1995;333(23):1517–21.
- Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ, 2000;321(7271):1259–62.
- Johnson S, Marriott L, Zinaman M. Can apps and calendar methods predict ovulation with accuracy? Current Medical Research and Opinion, 2018;34(9):1587–94.
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology, 2017;129(5):e150–4.
- Khambalia AZ, Roberts CL, Nguyen M, et al. Predicting date of birth and examining the best time to date a pregnancy. International Journal of Gynecology & Obstetrics, 2013;123(2):105–9.
- Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Human Reproduction, 2013;28(10):2848–55.
General information. Not a substitute for your doctor. These are findings from studies, not recommendations.