Many couples across the U.S. and around the world want to have children but they struggle with getting pregnant. There are many products that purport to detect peak fertility for prospective mothers, but many — such as basal temperature systems — are unreliable or ineffective. It turns out that your fitness band may be the best tool to determine when you have the best chance of getting pregnant.

The results of a study published by Nature indicate that resting pulse rate may vary predictably with a woman’s menstrual cycle phases. A study group of 91 women wore wristband devices that measured heart rate through a total of 274 ovulatory cycles. The researchers looked at the data collected when the subjects were asleep, as this reduced the possible influence of external factors, such as increased physical activity. The data showed a “significant increase” in the pulse rate during the fertility window, rising more than two beats per minute. During the luteal phase (between ovulation and menstruation), heart rate increased almost another two beats per minute.

The study found that the results were similar in spite of potentially confounding factors, such as coffee or alcohol consumption prior to sleep, exercise levels, or even body mass index (BMI). This could point to a low-cost, easily accessible, and reliable for prospective mothers to more accurately determine the best times for them to become pregnant.