For gestational diabetes, the risk was raised by 70% for those women who lived in the areas with heaviest traffic.
The study, Exposure to air pollution during pregnancy and health risks for mother and child, found that women in areas with poor air quality ran almost double the risk of developing gestational diabetes compared to women in areas with better air quality.
In the county of Skåne in southern Sweden, the ports, large cities and major roads experience heavy traffic and therefore elevated levels of air pollution. Nonetheless, the levels are usually within the permitted limits.
Air pollution linked to pre-eclampsia and diabetes
The study did not show any clear link between poor air quality and premature birth or low birth weight, as has been found in areas where air quality is considerably worse, such as in Los Angeles.
The study did, however, identify an air pollution association to three other conditions: pre-eclampsia, gestational diabetes and type 1 diabetes in children.
The strongest link was with gestational diabetes, where women experience high blood sugar during pregnancy. The risk was raised by 70% for those women who lived in the areas with heaviest traffic.
This is as high a risk as being overweight, which is often highlighted as a risk factor for gestational diabetes.
"Gestational diabetes is a relatively rare disease, so the risk for individuals is not very high. However, it can still have an impact on public health in general because of the large number of women affected", said Ebba Malmqvist, the author of the thesis.
It is not possible to say exactly how air pollution contributes to causing the three conditions, but experiments have shown that the small particles in polluted air can cause 'oxidative stress', which affects the immune system and leads to inflammation throughout the body.
We also know that this type of systemic inflammation is involved in both pre-eclampsia and type 2 diabetes (which often follows gestational diabetes), and that an over-reaction by the immune system is what causes type 1 diabetes.
"We also know that many of the particles in polluted air are so small that they can travel from the lungs into the blood. They thus affect the entire body and not only the respiratory system", said Malmqvist.
She would like politicians and authorities to consider lowering the limits for air pollution now that it has been shown that the permitted levels can have a negative impact on health.
About the study
Ebba Malmqvist's study is based on data from birth registers and diabetes registers for women who gave birth in Skåne between 1999 and 2005 (over 80,000 births).
This data has been combined with data on the levels of nitrogen oxides around the women's homes, traffic levels in the area and levels of ozone in Skåne.