NEW YORK (Reuters) -- Women who spend time in a hot tub or Jacuzzi during early pregnancy appear to have a two-fold higher risk of miscarriage, new research indicates.

However, not all experts are convinced, cautioning that there are important flaws in the way the study was conducted, which may have influenced the results.

Dr. De-Kun Li told Reuters Health that the results his team came up with should not be cause for panic among hot-tub users who are pregnant, since only a minority of women who spent time in a hot tub or Jacuzzi eventually experienced miscarriage.

But to err on the side of caution, the researcher, based at the Kaiser Foundation Research Institute in Oakland, California, recommended that women who are pregnant or trying to conceive stay out of a hot tub or Jacuzzi.

During the study, Li and his colleagues interviewed 1,063 recently pregnant women about their use of hot tubs or Jacuzzis. The researchers then followed the women and noted who experienced miscarriage before 20 weeks of pregnancy.

A total of 170 women eventually miscarried, 103 before the interviews took place.

Reporting in the ***American Journal of Epidemiology***, the researchers found that the risk of miscarriage appeared to increase among women who used the hot tub or Jacuzzi more than once per week, relative to less frequent users.

Miscarriage risk also appeared higher among users within the first four weeks of their last menstrual period than among later users.

Spending time in a regular bathtub, however, had no influence on miscarriage risk, according to the study.

Li explained that hot tubs and Jacuzzis may increase the risk of miscarriage by raising the body's temperature. To function properly, our bodies rely on proteins called enzymes, he noted, which are very sensitive to temperature. While adults have mechanisms in place to protect the body during rising temperatures, a growing fetus may not, making it "extremely sensitive to environmental changes," he added.

Indeed, Li noted that numerous studies have shown that spending time in a hot tub or Jacuzzi during pregnancy can increase the risk of birth defects.

Li added that increases in body temperatures during other activities likely do not put a growing fetus at risk. When women get hot from wearing heavy clothes or sitting in a sauna or steam bath, their bodies can cool off by sweating, he said, a mechanism that is not available when they are under water.