Use of Cycle Length Alone to Predict Ovulation, as Predicted by Apps, is Highly Inaccurate [19G]

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Women trying to conceive will often use methods to time intercourse. There are a plethora of free Apps available, claiming to be able to predict ovulation based on cycle length alone. This study sought to determine whether they could predict accurately.


This was an observational study of women >18 years old who were seeking to conceive (n=850). Volunteers collected daily urine samples for quantitative measurement of LH (AutoDELFIA), with day of ovulation determined as the day following LH surge. App determination of ovulation day for each cycle length was generated. Calendar based methods (rhythm, alternative rhythm, standard days and simple calendar method) were applied to determine accuracy.


For each cycle length there was a wide spread of possible days of ovulation. For a 28 day cycle, day 16 was most common (21%), range 11-20. The most accurate calendar method was the rhythm method which gave 89% accuracy, providing an average fertile window of 11 days, whereas the simple calendar method provided a 2 day window, but only achieved 22% accuracy.


The only way cycle length alone can be used to predict ovulation is by providing a very wide window of potential days, which is no better than the advice of frequent intercourse. Therefore Apps using cycle length alone are providing either inaccurate or imprecise information. If women wish to time intercourse to enhance their chance of pregnancy, they should use a true prospective method such as home ovulation tests.

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