Exactly what triggers the start of labor remains a mystery, but folklore holds that the weather may influence a baby's arrival. With Isaac -- now downgraded into a tropical storm -- making landfall, and two other storms brewing in the eastern Atlantic, some may be wondering if the Southeast is due for an increase in births.

Several studies have suggested that drops in barometric pressure can trigger either the onset of labor, or the rupture of the fluid-filled amniotic sac membrane, which is the technical term for a woman's water breaking or starting to leak.

"There's definitely a belief out there," said Dr. Jonathan Schaffir, associate professor of obstetrics and gynecology at Ohio State University College of Medicine. "It's certainly not cut-and-dried, but there is some scientific evidence that changes in pressure can contribute to membrane rupture," he said.

Of the studies that have looked at whether the weather might trigger pregnant women's water to break, two have suggested that it can, while two others have found no association, Schaffir said.

"The idea behind this belief is that the amniotic sac is like a balloon, and if you lower the external pressure on it, there is an increased risk it can 'pop,'" Schaffir said.

However, in his own experience as a practicing obstetrician for 18 years, Schaffir said he has seen no link between weather events and women going into labor. "In reality, the amniotic sac is protected. It's kind of hard to imagine that a small drop in barometric pressure would cause a change in the  amniotic sac," he said.

Dr. Salih Yasin, a practicing obstetrician for 25 years in Miami, also said he has not seen any increase in women going into labor during hurricanes.

As for the studies that have suggested a link, their usefulness in practice is doubtful, said Yasin, who is an associate professor of obstetrics and gynecology at the University of Miami Miller School of Medicine. The ranges of the barometric pressure changes in the studies were not very large, he noted.

In one study, researchers considered 162 women who, over the course of a year, went into labor at a Houston hospital around times of significant air pressure drops. Using air pressure data from the National Oceanic and Atmospheric Administration (NOAA), the researchers found that more women started labor after a drop in barometric pressure than prior to a drop, according to the study, published in 1997 in the Journal of Nurse-Midwifery. For example, for one drop in pressure, three women went into labor prior to it, but 11 began labor after it.

When looking at the 12 pressure drops that occurred that year, they found 66 women began labor prior to pressure drops, whereas 96 women began labor after the drops.

However, another study's results cast doubt on the link. In that 1996 study of about 2,400 pregnant women published in the American Journal of Obstetrics and Gynecology, the researchers found no link between days with the lowest pressure and the number of women who began labor.

But when the pressure drops were broken down into three-hour time periods, there was a link: fewer women went into labor during the hour after a period of falling air pressure.

"We were surprised to find a significant decrease in the onset of labor," after the pressure drops, the researchers wrote.

Yasin and a colleague looked at deliveries around Aug. 24, 1992, when the lowest barometric pressures drops during Hurricane Andrew were noted, at the University of Miami Jackson Memorial Hospital, which had the most deliveries in Miami-Dade County before, during and after Hurricane Andrew hit, Yasin said.

The researchers looked births and complications of pregnancy, and related them to NOAA data on barometric pressures, taking into account women's ZIP codes, to determine where they lived in relation to the Andrew's path. No association between air pressure and labor onset was found, Yasin said.

The most important things pregnant women can do during extreme weather events are to maintain their safety, eat and stay hydrated, he said.

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