By Lynne Peeples
NEW YORK (Reuters Health) - Pregnant women who live near busy roads may be at a greater risk for delivering before term, suggests a new study from Japan.
However, the researchers say it's too early to know for sure if traffic-related air pollution can actually cause preterm births.
Most babies are born after spending around 40 weeks in their mother's womb. About 13 percent of births in the U.S. are considered preterm, defined as delivery before 37 weeks, which may up the risk for health problems.
"Air pollution is considered to be a potentially important risk factor of preterm births," lead researcher Takashi Yorifuji, of Okayama University Graduate School of Medicine, told Reuters Health in an email.
But he added that few studies have evaluated the link for very premature births, which carry a particularly heavy public health burden, and none have looked at the effects for specific types of preterm births.
Yorifuji and his colleagues studied more than 14,000 babies born between 1997 and 2008 in Shizuoka, Japan. For each birth, they obtained detailed records on the pregnancy and how close to major roads the mothers lived.
Overall, 15 percent of women living within 200 meters (656 feet) from such a road gave birth to their baby before 37 weeks, while 10 percent of those living further away delivered prematurely, report the researchers in the journal Epidemiology.
Other factors have been tied to preterm birth, too, such as job, age and smoking. But after accounting for those, the team still found a 50-percent increase in preterm births among women living next to highly trafficked thoroughfares.
They also had a higher risk of delivering before 32 and 28 weeks, labeled 'severe' and 'extreme' prematurity, respectively.
"In addition, we found a higher risk in housewives than outside workers, and housewives would probably spend more time at home during their pregnancy, and reflect more accurate exposure," noted Yorifuji.
The researchers looked at two potential causes of prematurity: high blood pressure and early rupture of the membranes surrounding the fetus. Again, women living close to big roads had about double the risk of both.
Beate Ritz of the University of California, Los Angeles, who reviewed the study for the journal that published it, said the findings square with previous studies linking air pollution to high blood pressure and inflammation. That, in turn, might lead to premature rupture of the membranes.
"Everybody always worries that it's not really living by busy roadways, but that it's other things that makes these mothers different," she told Reuters Health.
"It's amazing that (the Japanese researchers) had enough data in their records to actually address the issue. After all the adjustments, the effect was still there," added Ritz. In her own research she has found a similar link, but she didn't account for all the other risk factors.
The one remaining issue the researchers didn't consider was noise pollution. "We can't rule it out," Ritz said.
The Japanese researchers also acknowledged that they lacked precise air pollution measurements. But Michael Brauer, of the University of British Columbia in Canada, said he wasn't too concerned.
"We know very well that living this close to roads of this level is linked to substantially higher levels of traffic-related air pollution at the home address," he told Reuters Health in an email.
Road proximity may be an especially useful measure since it is clear, easy to understand and can be directly related to land-use policy, he said.
So what is a pregnant woman to do if she finds herself living under an expressway or a block from a national highway?
"We find in our studies that it is quite common for mothers to move during pregnancy," said Brauer. "So as long as you are already considering moving, I would think about moving away from major roads."
If she can't avoid living near a busy thoroughfare, a pregnant woman may want to reduce the time she is active outside, as well as cut out smoking and improve her diet, added Yorifuji.
Ritz further recommended that pregnant women avoid traveling on busy roads, if at all possible.
"Certainly well-known modifiable risks should be reduced first," Brauer noted. "But I think the evidence is growing to indicate that this is a true risk that should be seriously considered along with the other more widely recognized risks."
SOURCE: http://link.reuters.com/xyk97q Epidemiology, online November 3, 2010.