Sorry TIME, the claims in your Period Tracker post are inaccurate

mm   in Pregnancy - June 12, 2016

A recent study claims that that the most popular Period Tracker apps and websites are inaccurate in tracking periods and fertility for women.

The study titled, “The Accuracy of Web Sites and Cellular Phone Applications in Predicting the Fertile Window” was published in the journal of Obstetrics & Gynecology and is authored by Dr. Robert Setton of the department of obstetrics and gynecology at Weill Cornell Medical College. The findings of this study have attracted a lot of coverage on the media including TIME, NIH MedlinePlus, etc.

We believe that the methodology used to measure the accuracy of period tracking apps and websites is incorrect and therefore cannot be used to make such claims.

Setton Study Summary

Dr. Setton and his colleagues identified the most popular 33 apps and 20 websites to measure their accuracy.

As reported by the TIME article,

“For each of the apps, Setton and his colleagues plugged in a typical cycle of 28 days with four days of menses. They set the last menstrual period to January 1, which put the date of ovulation on January 15, and the fertile window was January 10–15. If the apps matched the fertile window calculated by the researchers, they deemed the app accurate.”
Then they tested these 33 apps and 20 websites using above method which resulted in only 3 apps and 1 website predicting the exact fertile window. Thereby, Dr. Setton and his colleagues concluded that most of the apps and websites currently in the market are inaccurate.

Assumptions that render the claims incorrect

Variability in length of cycles
First, the Setton study uses 28 days as a standard menstruation cycle length of 28 days to test the accuracy of all apps and websites. However, there is a lot of variability in the length of cycles from women to women and even from cycle to cycle for the same woman due to various factors.

A well known Marquette study by Fehring, evaluated the variability of different phases of menstrual cycle, was published in the Journal of Obstetric, Gynecologic, and Neonatal Nursing studied 141 healthy women and collected 1665 cycles of data.

We analyzed the 1665 cycles of data and found that the cycle lengths can vary anywhere from 18 to 54 days. When we look at the cycles of the 141 women that participated in the study only 14% of cycles were 28 days in length.

Another large study that collected data from 2,316 women and recorded data of 30,655 cycles reported that most of the cycle lengths varied between 15 and 45 days.

Variability in the day of ovulation
Second, the Setton study uses cycle day 15 as the day of ovulation to calculate the fertile window. Similar to the length of cycles, there is a lot of variability on the day of ovulation due to various factors.

Assuming that all women and all cycles of every woman have a length of 28 days, we identified all the cycles in the same Marquette study data that had a cycle length of 28 days. We identified 218 such cycles and analyzed the variability of the day of ovulation and found that the day of ovulation can vary from as soon as cycle day 10 to as late as cycle day 20.

As shown in the above chart, there was almost equal percentage of women’s cycles that had ovulation on day 13, 14, 15 or 16. We can see that 80% of women ovulate anywhere between cycle days 13 to 16. This indicates that assuming women with 28 day cycles ovulate on day 15 is not completely accurate.

Variability in fertile window
There is research that talks about the variability in the number of fertile days from one woman to the other due to various factors like age, lifestyle, weight, etc. However, the Setton study authors have provided good rationale on why to assume that fertile window is 6 days long ending on the day of ovulation. Therefore, if we assume fertile days are the 5 days preceding ovulation plus the day of ovulation as the study does, then fertile days for the women in Marquette study would be anywhere from cycle day 5 to cycle day 20, which is a 16 day fertile period.

Another study by Wilcox, the same author referenced by the Setton study, provides estimates of the likely occurrence of the 6 day fertile window during the menstrual cycle and concluded with the same assessment,
“In only about 30% of women is the fertile window entirely within the days of the menstrual cycle identified by clinical guidelines. Most women reach their fertile window earlier and others much later. Women should be advised that the timing of their fertile window can be highly unpredictable, even if their cycles are usually regular.”

Estimates alone cannot predict fertile period

We agree with Dr. Setton and his colleagues that women who are trying to conceive should be made aware of the inaccuracies of the calendars generated by fertility apps and websites, and that estimates alone cannot be used to accurately predict fertile window.

Solutions exist in the market

Accurate solutions exist that remove the unpredictability of the day of ovulation or fertile windows due to the variability that we discussed here. One of the best solutions for predicting ovulation is Ultrasound and is routinely used by Fertility Specialists to predict ovulation. However, ultrasound is time-consuming and cost prohibitive. Who wants to go into an Obstetrician or Gynecologists’ office every day to check for ovulation.

There is another solution that has been shown to be highly accurate in predicting ovulation day and it involves measuring Luteinizing Hormone LH from urine or blood (Study 1, Study 2).

That is why we developed a smartphone connected fertility monitor that measures fertility hormones from urine and accurately predicts ovulation day and fertile periods. It removes the guesswork that a lot of app and website based period trackers rely on. We can predict ovulation 12–48 hours in advance using our unique sensor to help women become pregnant.

Women are unique

Women are unique, their bodies are unique and so are their menstrual cycles. Trying to define women’s cycles as 28 days long and ovulation on day 15th is one of the fundamental fallacies of the last 50 years that we the next generation of fertility and period trackers are working to change.


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