A woman's heart rate appears to increase slightly when she's at the most fertile point in her menstrual cycle, according to a new study that's one of the first to use wearable devices to detect this change.

Researchers found that during a woman's fertile window — the period of about six days when a woman can become pregnant — her resting heart rate increases by about 2 beats per minute, on average, compared with her heart rate during menstruation.

The findings suggest that heart rate could help detect the start of a woman's fertile window in real time using wearable sensors, according to the researchers, from the medical technology company Ava. The company makes a wearable device called the Ava bracelet, which is marketed to detect the fertile window.

However, the researchers stressed that women should not use heart-rate data by itself to detect the start of their fertile window. The company suggests that other measurements, such as skin temperature and sweating patterns, more accurately determine the start of the fertile window. [Conception Misconceptions: 7 Fertility Myths Debunked]

In addition, experts not involved in the study had concerns about the accuracy of this method if it is not used consistently, and pointed to the need for more research on whether scrutinizing heart-rate data can indeed increase a woman's chances of becoming pregnant, compared with other methods.

Heart rate and fertility

Some previous studies have found a link between heart rate and women's fertile window. For example, a study published in 2000 found an increase in women's resting heart rate during ovulation (the time when the ovaries release an egg), but this study was conducted in a lab clinic, took measurements only five days a week, and followed participants for only a single menstrual cycle. In addition, the study took place during the daytime, when a person's heart rate is more variable (based on their emotions, for example.) than it is during the night when a person is asleep, the Ava researchers said.

In the new study, the researchers analyzed information from 91 healthy women ages 22 to 42 who were not pregnant. Participants wore a heart-rate monitoring device on their wrists (either the Ava bracelet or another wrist-worn heart-rate monitor) at night while they slept. On average, the women wore the device for three complete menstrual cycles. The women also used a home urine test to estimate the day when they were ovulating.

The study found that, on average, the women's heart rates increased by 2.1 beats per minute during their fertile window — which includes the day of ovulation and five days before ovulation — compared with their heart rate during menstruation.

The women's heart rates continued to increase during the luteal phase, which occurs after ovulation but before a woman's period starts. During this phase, the women's heart rates increased by about 3.5 beats per minute, compared with their heart rate during menstruation.

Factors such as drinking alcohol, eating large meals or exercising within 4 hours of bedtime also increased a woman's resting heart rate at night. But the study's findings held even after the researchers took into account these factors — the increase in heart rate seen during the fertile window occurred in addition to the increases due to these other factors, the researchers said.

Potential concerns

Dr. Mamie McLean, an obstetrician/gynecologist and infertility specialist at The University of Alabama at Birmingham School of Medicine, who was not involved with the study, said that monitoring the heart rate using wearable sensors appears to be a promising method for predicting a woman's fertile window.

However, McLean said that, since the heart rate rises during both ovulation, when a woman is more likely to get pregnant, and the luteal phase, which comes immediately after this time, there is a concern that a woman could detect a change in her heart rate at the wrong point in her cycle if she doesn't wear the tracker every night.

"If a female doesn't wear it every single night, she may miss the window of ovulation and incorrectly find the increased heart rate during the luteal phase rather than ovulation phase," McLean said. During the luteal phase, a woman cannot become pregnant. [Trying to Conceive: 12 Tips for Women]

In addition, the study mostly involved Caucasian women with a normal body mass index (BMI), so it's not clear how well the findings would apply to a more diverse population.

More studies are needed to determine how pregnancy rates with this method compare to those with other methods, such as timed intercourse or urine testing, McLean said.

It's also important to note that women should not use heart-rate changes as a method of contraception to avoid pregnancy, McLean said, because more evidence is needed to show that heart-rate monitoring could work for that purpose.

Still, McLean was hopeful that with further research, this wearable method could become another option for women trying to conceive. "I think it's exciting and I'm hopeful this will provide patients with another method of detecting the fertile window," she said.

The study was published online May 2 in the journal Scientific Reports.

Original article on Live Science.