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Pregnant women who perform exercises that push their heart rate beyond 150 beats a minute may increase their likelihood of suffering a miscarriage, according to a study by Aichi Medical University.

A pulse rate exceeding 150 can cause irregularities in a fetus’ heart rate, which has been linked to premature labor and miscarriage, according to the research team led by Mitsuoki Asai, an associate professor of obstetrics and gynecology at Aichi Medical University.

The finding is to be presented at a meeting of the Japan Society of Obstetrics and Gynecology next month in Fukuoka.

Doctors say pregnant women who engage in prenatal fitness exercises, including swimming aerobics, could see their heartbeats exceed 150 a minute.

The Aichi study has already prompted the Japanese Society of Clinical Sports Medicine to plan guidelines advising pregnant women to watch their heart rates when they exercise.

The team began the study in 1989 and measured the heartbeats of 585 pregnant women exercising on cycling machines. Only women more than five months into pregnancy were chosen for the study.

The subjects were told to pedal until their pulse rate hit about 160, and doctors observed the heart rate of their fetuses by ultrasound a minute later.

As the heart of a fetus tends to beat slower as it grows, the researchers divided the heart rates of the fetuses into two groups depending on the length of pregnancy: fast pulses when the heart beat faster than 160 to 166 per minute, and slow when it was below that level, before the start of exercise.

The study found that 24 fetuses showed symptoms of rapid pulses, while seven showed a slow pulse. The researchers said these symptoms were observed only among pregnant women who developed a pulse rate exceeding 150.

The researchers found no irregular fetal heartbeats when the women’s pulse rate stayed below 150.

Asai said that while there is no immediate danger, pregnant women should exercise with caution and check with their doctors before undertaking any rigorous exercise.

Yasuhiro Tanaka, a gynecologist affiliated with the Japan Maternitybics Association, said pregnant women should try to avoid excessive exercise and try to keep their heartbeats within 130 to 150 pulses when exercising.

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