Pregnant women have another thing to be wary of while carrying their child through to full term-- the air they breathe. Researchers found exposure to high levels or air pollution during the second trimester of pregnancy may greatly increase the children’s chances of having asthma, and will present the new study at the 2014 American Thoracic Society International Conference this week.

"We know that mothers' exposure to air pollution during pregnancy can affect lung development of their babies and lead to subsequent respiratory disorders, including asthma, although little is known about whether timing of the exposure is important to consider," said Dr. Yueh-Hsiu Mathilda Chiu, lead author from the Department of Pediatrics at Icahn School of Medicine at Mount Sinai in New York.

In order to shed more light on the relationship between pollution inhalation and asthma, researchers followed 430 children who were born to full term up until the age of seven, along with their mothers. Where each mother resided at the time of pregnancy was recorded, in order for researchers to estimate the amount of air pollution they were exposed to while they were pregnant.

"In our study, we assessed whether higher exposure to particulate air pollution at more specific time windows in pregnancy were particularly linked to higher asthma risk in urban children," said Chiu.

Daily air pollution from traffic power plants, and even farmlands needed to be taken into account because of the fine particles that are released into the air, which makes them more likely to be inhaled deeply into the lungs. Those tiny particles have been linked as a health risk to pregnant women, as past studies have suggested the effects can be transferred to their gestating baby. According to the Centers for Disease Control and Prevention, an estimated 6.8 million children have asthma in the United States.

"While we should continue to improve air quality and minimize exposure to pregnant women throughout the entire pregnancy for a host of health reasons," said Dr. Rosalind Wright, senior investigator from the Department of Pediatrics at Icahn School of Medicine at Mount Sinai. "Pinpointing the gestational period during which air pollution has the greatest effects on the developing lung may add to our understanding of the mechanisms underlying this relationship."

Women in their second trimester, which is the 13th to the 28th week of pregnancy, who were exposed to higher levels of fine, yet dangerous, air particles had the most increased risk of having a child with asthma. The association was stronger when the child was born to a non-obese mother.

According to researchers, there are also additional health risks mothers-to-be must make note of, such as obesity. "It is possible that the effect of maternal obesity, another known risk factor of childhood asthma onset, may be so strong that it was difficult to determine additional effects of air pollution among children born to obese mothers in this setting."

The relationship between asthma and obesity has been known since the National Institute of Public Health and the Environment in the Netherlands took a look at 4,000 children with one out of five mothers being overweight. Those who had at least one asthmatic parent and were born from an overweight mother had a 65 percent greater chance of having asthma than those born from normal weight mothers.

The next step for researchers will be to understand why lung growth and airway development in the second trimester is so greatly affected by the air their mother breathes. Researchers will also explore how maternal obesity affects the child’s lungs and the increased risk of asthma.

Chiu, Y, Wright R, et al. Identifying Prenatal Windows Of Susceptibility To Particulate Air Pollution On Childhood Asthma Onset In A Prospective Urban Birth Cohort. American Thoracic Society International Conference. 2014.