Wilcox et al studied the probability that a woman would be in her 6 day fertile window on a specific day in her menstrual cycle. The timing of the fertile window can be highly unpredictable, with only a third of women having the fertile window entirely between days 10 and 17. The authors are from the National Institute of Environmental Health Sciences in Durham, North Carolina.
Parameters:
(1) cycle regular vs irregular
(2) if regular, the number of days in the cycle

Probability of Being in the Fertile Window 


day of cycle 
irregular cycle 
regular cycle < 28 days 
regular cycle of 28 days 
regular cycle of 29 days 
regular cycle >= 30 days 
0 
0.000 
0.000 
0.000 
0.000 
0.000 
1.75 
0.000 
0.002 
0.001 
0.001 
0.001 
3.5 
0.002 
0.025 
0.022 
0.019 
0.009 
5.25 
0.013 
0.119 
0.075 
0.056 
0.028 
7 
0.041 
0.350 
0.216 
0.134 
0.063 
8.75 
0.100 
0.616 
0.475 
0.375 
0.172 
10.5 
0.194 
0.703 
0.631 
0.566 
0.363 
11.5 
0.269 
0.688 
0.663 
0.628 
0.450 
12.25 
0.300 
0.631 
0.656 
0.644 
0.503 
12.8 
0.341 
0.541 
0.625 
0.656 
0.541 
14 
0.388 
0.398 
0.519 
0.594 
0.556 
15.75 
0.428 
0.219 
0.313 
0.428 
0.475 
17.5 
0.409 
0.128 
0.194 
0.288 
0.350 
19.25 
0.356 
0.075 
0.122 
0.175 
0.228 
21 
0.291 
0.050 
0.074 
0.097 
0.153 
22.75 
0.211 
0.038 
0.044 
0.060 
0.106 
24.5 
0.156 
0.034 
0.031 
0.041 
0.081 
26.25 
0.116 
0.038 
0.025 
0.024 
0.063 
28 
0.091 
0.049 
0.031 
0.022 
0.063 
29.75 
0.075 
0.056 
0.044 
0.019 
0.064 
31.5 
0.066 
0.069 
0.056 
0.019 
0.069 
33.25 
0.069 
0.075 
0.075 
0.021 
0.088 
35 
0.077 
0.081 
0.103 
0.026 
0.103 
approximated from Figures 2 and 3, pages 12601261
Approximating Equations from JMP: Irregular Cycle
probability in fertile window for day 0 to 11.5 =
= (0.0002876 * ((day of cycle) ^3)) – (0.001458 * ((day of cycle) ^2)) + (0.002127 * (day of cycle)) – 0.000208
probability in fertile window for day 11.5 to 19.25 =
= (0.000122 * ((day of cycle) ^3)) – (0.001685 * ((day of cycle) ^2)) + (0.1520585 * (day of cycle)) – 1.075533
probability in fertile window for day 19.25 to 29.75 =
= (0.0000726 * ((day of cycle) ^3)) – (0.003195 * ((day of cycle) ^2))  (0.002957 * (day of cycle)) + 1.0822857
probability in fertile window for day 29.75 to 35 =
= (0.000218 * ((day of cycle) ^3)) + (0.0225306 * ((day of cycle) ^2))  (0.772476 * (day of cycle)) + 8.847
Approximating Equations from JMP: Regular Cycle of < 28 Days
probability in fertile window for day 0 to 8.75 =
= (0.000463 * ((day of cycle) ^3)) + (0.006471 * ((day of cycle) ^2))  (0.021504 * (day of cycle)) + 0.0046984
probability in fertile window for day 8.75 to 12.25 =
= (0.003236 * ((day of cycle) ^3)) + (0.0759629 * ((day of cycle) ^2))  (0.510844 * (day of cycle)) + 1.4375909
probability in fertile window for day 12.25 to 21 =
= (0.000491 * ((day of cycle) ^3)) + (0.0334244 * ((day of cycle) ^2))  (0.761041 * (day of cycle)) + 5.8401134
probability in fertile window for day 21 to 28 =
= (0.000016 * ((day of cycle) ^3)) + (0.0024023 * ((day of cycle) ^2))  (0.089667 * (day of cycle)) + 1.0175857
probability in fertile window for day 28 to 35 =
= (0.000093 * ((day of cycle) ^3)) + (0.0086064 * ((day of cycle) ^2))  (0.258776 * (day of cycle)) + 2.595
Approximating Equations from JMP: Regular Cycle of 28 Days
probability in fertile window for day 0 to 8.75 =
= (0.0011368 * ((day of cycle) ^3)) – (0.0046 * ((day of cycle) ^2)) + (0.0075616 * (day of cycle)) – 0.000921
probability in fertile window for day 8.75 to 14 =
= (0.000583 * ((day of cycle) ^3)) – (0.003965 * ((day of cycle) ^2)) + (0.3287014 * (day of cycle)) – 1.707444
probability in fertile window for day 14 to 26.25 =
= (0.000402 * ((day of cycle) ^3)) + (0.0290718 * ((day of cycle) ^2))  (0.706321 * (day of cycle)) + 5.808474
probability in fertile window for day 26.25 to 35 =
= (0.0000587 * ((day of cycle) ^3)) – (0.004615 * ((day of cycle) ^2)) + (0.1252305 * (day of cycle)) – 1.144889
Approximating Equations from JMP: Regular Cycle of 29 Days
probability in fertile window for day 0 to 10.5 =
= (0.0003991 * ((day of cycle) ^3)) + (0.0020603 * ((day of cycle) ^2))  (0.010923 * (day of cycle)) + 0.005
probability in fertile window for day 10.5 to 15.75 =
= (0.0009901 * ((day of cycle) ^4)) – (0.051103 * ((day of cycle) ^3)) + (0.9607751 * ((day of cycle) ^2))  (7.796999 * (day of cycle)) + 23.633397
probability in fertile window for day 15.75 to 29.75 =
= (0.000219 * ((day of cycle) ^3)) + (0.0183492 * ((day of cycle) ^2))  (0.512868 * (day of cycle)) + 4.8137222
probability in fertile window for day 29.75 to 35=
= (0.0000311 * ((day of cycle) ^3)) – (0.002612 * ((day of cycle) ^2)) + (0.0724762 * (day of cycle)) – 0.644
Approximating Equations from JMP: Regular Cycle of >= 30 Days
probability in fertile window for day 0 to 11.5 =
= (0.0004607 * ((day of cycle) ^3)) – (0.001963 * ((day of cycle) ^2)) + (0.0019073 * (day of cycle)) + 0.0014143
probability in fertile window for day 11.5 to 15.75 =
= (0.000406 * ((day of cycle) ^3)) – (0.004533 * ((day of cycle) ^2)) + (0.357417 * (day of cycle)) – 2.444863
probability in fertile window for day 15.75 to 29.75 =
= (0.00015 * ((day of cycle) ^3)) + (0.0135822 * ((day of cycle) ^2))  (0.408112 * (day of cycle)) + 4.1223968
probability in fertile window for day 29.75 to 35 =
= (0.0008163 * ((day of cycle) ^2))  (0.045086 * (day of cycle)) + 0.6819
Specialty: Obstetrics & Gynecology
ICD10: ,