Whether a woman is trying to get pregnant or avoid pregnancy, it's likely she's tried using a period-tracking or fertility app.

Mobile apps that track your menstrual cycle are used by tens of millions of women, but how accurate they are remains unclear.

Dr. Jessica Chan, a Fertility and Reproductive Medicine expert at Cedars-Sinai, says that all menstrual cycle apps use approximate dates to figure out when a woman might be ovulating.

"Unless someone is checking ovulation through physical means, like by using an ovulation prediction kit, the cycle app is just providing an estimate of when their fertile days are," Dr. Chan says.

In fact, a 2018 study found that the accuracy of prediction by menstrual cycle apps was no better than 21%.

Read: What You Need to Know About Your Fertility

According to the American Pregnancy Association, a woman's menstrual cycle averages between 28-32 days. Starting with the first day of the last menstrual period, most women ovulate anywhere between day 11 and day 21.

"For the most part, women do ovulate right in the middle of their cycles," Dr. Chan says. "But for a lot of women, even though they are having regular periods and a regular cycle, there are various factors that could lead to lack of ovulation each month, such as hormone problems."

While fertility apps may not be able to accurately predict ovulation, Dr. Chan says they can help women become more aware of their menstrual cycle—especially if they want to get pregnant.

"Before they are trying to get pregnant, people don't really keep track," Dr. Chan says. "When they bring the data collected from the fertility app to me, it's really useful. I can see how long their cycles are."

Read: Understanding What Women Want During Childbirth

If a woman or couple is trying to optimize fertility, using an ovulation predictor kit is the best method, says Dr. Chan. 

According to the World Health Organization, home-based use of ovulation predictor kits may improve fertility management when attempting to become pregnant. In some studies, evidence shows that ovulation predictor kits may increase pregnancy rates by about 40%.

Dr. Chan says one of the biggest misconceptions about getting pregnant is that pregnancy can be achieved after only a few tries. Sometimes it can take a few months or even longer. 

There are factors in both men and women that can cause infertility.

"If there's something causing infertility, like poor sperm, blocked fallopian tubes, or advanced age, none of these things can be fixed with an app," Dr. Chan says. "There's a limit to the amount of information that technology can provide before you need a real, complete medical evaluation."

A couple is defined as infertile if conception does not happen after 1 year of unprotected sex for women under age 35, and after 6 months of unprotected sex for women over age 35.

"If you're still not pregnant after 6-12 months of trying, definitely make an effort to see a fertility doctor," Dr. Chan says. "If there are other aspects of your medical history that might put you at risk for infertility, such as irregular or no periods, or a history of endometriosis, you should be seen even sooner." 

While apps can help women be more aware of their menstrual cycle, it's important to regularly see a physician to discuss your birth control options and family planning.