Websites and apps that promise to calculate a woman's most fertile days may often be off base, a new study suggests.
When doctors put 53 fertility calculators to the test, they found that only four accurately predicted a hypothetical woman's "precise fertile window."
"I'd recommend that consumers be cautious, and not completely rely on these sites and apps," said lead researcher Dr. Robert Setton. He is an obstetrics and gynecology resident at New York-Presbyterian Hospital/Weill Cornell Medical College, in New York City.
Setton was to present the findings Sunday at the annual meeting of the American College of Obstetricians and Gynecologists, in Washington, D.C. Such research is considered preliminary until published in a peer-reviewed medical journal.
The concept behind online fertility calculators is simple, Setton explained. A woman typically needs to know the date of her last menstrual period, and the average length of her cycles. The calculator then tells her which days she'll have the best chance of conceiving.
There are many such services out there, according to Setton. "But to the best of our knowledge, no one has studied their accuracy," he said.
His team assessed 20 websites and 33 apps that were the top results in an online search. The researchers put the same information into each fertility calculator: the same date for the last menstrual period, and an average menstrual cycle length of 28 days.
According to Setton, research shows that a woman's "fertile window" -- or the best time to have sex -- includes the day she ovulates and the five days before that. For a woman with a menstrual cycle of 28 days, that would be days 10 through 15.
Yet only one website and three apps came up with that fertile window, Setton said. They included the Babymed.com site and the apps Clue; My Days - Period & Ovulation; and Period Tracker, the study found.
The rest of the calculators, Setton said, "were all over the place."
Some gave fertile windows that were 10 days long, he noted, and some incorrectly predicted the day of ovulation.
What's not clear is whether any of these calculators are hindering couples from conceiving.
"If the average woman without fertility issues uses one of these, will it affect her ability to get pregnant?" Setton said. "We don't know."
But, he added, the concern is that some couples, especially those having trouble conceiving, will rely on these tools.
Dr. Jennifer Wu, an obstetrician-gynecologist at Lenox Hill Hospital in New York City, agreed.
"Many patients use fertility apps on their smartphones. They do need to be aware that many of these apps are inaccurate," said Wu, who was not involved in the study.
She noted that this study used an average menstrual cycle length of 28 days. "But it's actually normal for cycle length to vary from 21 to 35 days," she said.
Wu suggested that women trying to conceive talk to their doctor about their individual cycle length, and how it varies from month to month.
In some cases, Wu said, women might benefit from an ovulation prediction kit. Those are home-based electronic monitors that gauge hormone levels in the urine or saliva.
Setton made another point: There are some people who use fertility-prediction sites and apps to avoid pregnancy.
"Definitely don't rely on them for that," he advised.