Most fertility-tracking websites and phone apps provide women with inaccurate information on the best time to conceive, a study has found.

Out of 53 websites and apps, only one website and three apps predicted the same fertile window as researchers using the 'gold standard' recognised by fertility experts.

'Doctors I work with were really surprised,' Dr Robert Setton of Weill Cornell Medical College, who was author of the new study, told Time magazine. 'We all assumed that these are fine and probably accurate, because how could you mess something like this up? It's so simple.'

Writing in the journal Obstetrics and Gynecology, Dr Setton and colleagues describe how they found the websites and apps through a search for the terms 'ovulation calendar' and 'fertility calendar'.

To calculate the theoretical fertile window, they used a widely respected model based on a 1995 study of over 200 healthy women. According to this model, the fertile window is the date of ovulation and the preceding five cycle days.

Assuming a 28-day cycle, ovulation would be expected to occur on day 15. However, 26 out of the 33 apps and 15 out of the 20 websites analysed included days after ovulation in the fertile window, even though women are unlikely to conceive at that point. The websites and apps varied hugely in the fertile windows they predicted: some started as early as day 4 while others went on as late as day 21.

Although not all of the apps predicted the day of ovulation, most of those that did - 85 percent - were accurate.

'Although for this study we assumed a "perfect cycle", it can be implied that with the inherent variation in actual cycles, the predicted fertile windows may be even more inaccurate,' the authors write. 'This all may lead to patients having intercourse in patterns that will not maximise their chances of conceiving.'