Pregnant women who perform exercises that push their heartbeat beyond 150 pulses per minute may cause irregularities in the fetus heartbeat, according to a study conducted by Aichi Medical University.
The finding, made by a research team led by Mitsuoki Asai, associate professor of obstetrics and gynecology at Aichi Medical University, was presented at a meeting of the Japan Society of Obstetrics and Gynecology in Fukuoka.
While irregular heartbeats in fetuses have been linked to premature labor and miscarriage, doctors say it is not unusual that pregnant women who perform fitness exercises, such as swimming aerobics, develop heartbeats that exceed 150 pulses per minute.
The Aichi Medical University study has already prompted the Japanese Society of Clinical Sports Medicine to issue guidelines this fall advising pregnant women to watch their heartbeats when they exercise.
The Asai team began the study in 1989 and measured the heartbeats of 585 pregnant women exercising on cycling machines. Only women in stable pregnancy, that is, those who are more than five months, were chosen for the study.
The subjects were told to pedal cycling machines until their heartbeats rise to about 160 pulses per minute, and doctors observed the fetus heartbeats through ultrasonic sensors one minute later.
As the fetus heartbeat tends to move slower as the fetus grows, the researchers divided the heartbeats of the fetuses into two groups depending on the length of pregnancy: fast pulses, when the heartbeats rise to more than 160 to 166 per minute; and slow pulses, when the heartbeats slow down below the level before the start of exercise.
The results of the survey show 24 fetuses showed symptoms of fast pulse, while seven showed symptoms of slow pulse. The researches say these symptoms were observed only among pregnant women who developed heartbeats exceeding 150 pulses per minute.
The researchers found no symptoms of heartbeat irregularities in fetuses when the pregnant women's heartbeat stays under 150 pulses.
Asai says while there is no immediate danger for pregnant women with heartbeats exceeding 150, they should exercise with caution and check with their doctors before undertaking any vigorous exercise.
Yasuhiro Tanaka, a gynecologist associated with the Japan Maternity Aerobics Association, said pregnant women should try to avoid excessive exercise and try to keep heartbeats within 130 to 150 pulses when exercising.
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