The findings add to evidence that air quality may have subtle effects on reproduction. Several studies have linked exposure to air pollution during pregnancy to an increased risk of preterm delivery and low birthweight.

 

It remains unclear, however, whether poor air quality itself is responsible. These latest findings, from a study of 7,400 U.S. women who underwent IVF over seven years, point to a complex relationship.

 

One common pollutant — nitrogen dioxide (NO2), largely produced by vehicle exhaust — was consistently linked to lower odds of IVF success. When NO2 levels were higher-than-average near a woman's home, or near the IVF lab, during any parts of the IVF process or during pregnancy, the chances of having a baby dipped.

 

The findings were more complicated when it came to another major pollutant — ozone.

 

Higher-than-average ozone levels after embryo implantation in the womb were associated with decreased odds of ultimately having a baby. However, higher ozone exposure around the time of ovulation was connected to improved chances of having a baby.

 

The reasons for the disparate findings on ozone are not clear, according to the researchers. But the positive effects associated with ozone exposure may reflect "confounding" by NO2 levels, said lead researcher Dr. Richard S. Legro, of Pennsylvania State University College of Medicine in Hershey.

 

That is, NO2 levels tend to decline as ozone levels rise, and NO2 was the pollutant consistently linked to lesser IVF success in this study. "This argues that high levels of nitrogen dioxide are the bad one," Legro said.

 

He stressed, however, that women who are pregnant or planning on becoming pregnant, naturally or by IVF, "shouldn't panic." The association between NO2 levels and IVF outcomes in this study was statistically significant — meaning it is unlikely it was a chance finding — but translated into real life, effects of the pollutant would be "subtle," Legro said.

 

The study, published in the journal Human Reproduction, included 7,403 women who underwent IVF at one of three fertility clinics between 2000 and 2007; the centers were located in Hershey, New York City and Rockville, Maryland, representing rural, urban and suburban areas.

 

Legro said his team focused on IVF patients because the researchers would know precisely when each woman ovulated, when fertilization occurred and when embryos were implanted in the uterus. That allowed them to look at the relationship between air pollution levels and each step of the pregnancy process.

 

The researchers used data from government air-quality monitors near each study participant's home and the fertility centers, collected over the seven-year study period.

 

Of the whole study group, 36 percent of the women had a baby following their first IVF treatment. Overall, Legro's team found that those odds dipped by 20 percent when the NO2 levels near a woman's home were 0.01 parts per million above average at the time she was taking medication to spur ovulation.

 

Similar effects were seen when the researchers focused on NO2 levels near the IVF center at the time of egg retrieval and fertilization, and levels near the women's homes after the embryos had been implanted. Although IVF labs are tightly controlled environments, their indoor air quality still varies, Legro said.

 

Still, while the study points to an association between air pollution, particularly NO2, and IVF outcomes, it does not prove cause-and-effect.

"We can't show a mechanism," Legro said. "We can't say that (for example,) poor air quality leads to poor egg quality."

 

And a key limitation of the study, he and his colleagues note, is that it did not have direct measurements of the women's personal exposure to various air pollutants.

 

"We still need to do further studies and confirm these findings," Legro said. "It's too soon to say what the ultimate effects (of air pollution) on reproduction are."

 

In theory, high levels of air pollutants could affect pregnancy outcomes for several reasons, according to Legro. Air pollution might cause widespread inflammation in the body, increase the body's production of cell-damaging oxygen-free radicals, or make the blood more prone to clotting — all of which could pose a risk to pregnancy.