Pregnant women who enjoy relaxing in the hot tub shouldn't stay in more than 10 minutes, says a study published Thursday.

Too much tub time can raise a woman's body temperature enough to cause malformations in the fetus, said the study, appearing as a letter in the New England Journal of Medicine.According to the latest research, pregnant women should not spend any more than 10 minutes in a tub when the water is 104 degrees.

To reach this conclusion, the researchers conducted an experiment on 24 young, non-pregnant women to see how long it takes for their inner-body temperature to reach 102, the point that can be hazardous to the fetus.

They found that some people heat up twice as fast as others. In a 104-degree tub, some women's bodies reached 102 in as little as 12 minutes, while others took 23 minutes.

They also found that half of the women did not feel uncomfortably hot when their temperatures hit 102.

"We conclude that at least in non-pregnant women, subjective discomfort is not wholly reliable as a safeguard against overheating and that time limits should also be used," the researchers wrote.

The study was conducted in Australia by Barry R. Ridge of the University of Western Sydney-Macarthur and Dr. Grahame M. Budd of the National Institute of Occupational Health and Safety in Sydney.