Women who turn to the internet and mobile apps to sort out the best time of the month to try getting pregnant may receive bad advice, a study suggests.

Researchers tested 20 websites and 33 apps designed to help predict what's known as the fertile window, the days before ovulation when having sex is most likely to result in conception.

Typically, a woman with a standard 28-day menstrual cycle will ovulate around day 15, which would also be the last day of a six-day fertile window.

When researchers asked these apps and websites to give a fertile window for a woman with a 28-day cycle, most of them reliably predicted the day of ovulation, the study found. But only four provided the correct fertile window.

"Before using any website or app, women need to understand that the actual fertile window consists of the day of ovulation plus the preceding five cycle days," said lead study author Dr. Robert Setton, a researcher at Weill Cornell Medicine and New York Presbyterian Hospital.

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"They can use the app or website to help them predict their ovulation date and then use that as a guide for the rest of the fertile window," Setton added by email.

Setton and colleagues tested all of the websites and apps with the same case: a woman with a 28-day menstrual cycle and four days of menses whose last period started January 1.

In this test case, 80 percent of websites and 87 percent of apps that predicted the day of ovulation correctly said it would occur on January 15, the study found.

But just one website and three apps correctly predicted a fertile window of January 10-15.

The only accurate website for the fertile window was babymed.com, Setton said. Among the apps, the ones that got it right were iPeriod, My Days and Clue.

The findings suggest that women should be cautious about relying only on websites and apps to predict the best days each month to try to conceive, the authors conclude in Obstetrics and Gynecology.

Couples using an incorrect fertile window from an app or website to time intercourse may end up having sex too soon or too late in the month to conceive, the authors note.

One limitation of the study is that researchers only included free apps and websites, the authors acknowledge. The study also didn't examine how often couples decide to have sex based on the fertile window suggested by these tools, or explore how often couples conceived.

Even so, the findings suggest that women may want to rely on alternatives to determine their fertile window each month, said Deborah Lupton, a researcher at the University of Canberra in Australia who wasn't involved in the study.

Women can do this by tracking some changes in their bodies that occur around ovulation, including a slight spike in body temperature and an increase in vaginal discharge as the cervix releases thin, clear mucus.

"Once women do this for a while, they should get a good sense of where in their cycle they ovulate," Lupton said by email.

"These apps and software tools are not likely to be effective for women who either are trying to conceive or using these tools to avoid conception," Lupton added.