Pollution linked to smaller babies

‘Mothers living near busy roads “have smaller babies,” The Times has reported. The newspaper said a new study found that exposure to traffic pollution could affect fetal development and lead to an increased chance of having a small baby.

The study looked at 336,000 babies born in New Jersey between 1999 and 2003 and found that the higher a mother's level of exposure in early and late pregnancy, the more likely it was that the baby would not grow properly, even after taking into account a range of known risk factors for small babies. These factors included mother’s age, poor education, poverty, smoking and being a single parent, which all also increased the risk of a small baby in this study.

This was a large and carefully conducted study, which suggests pollution is another risk factor for impaired fetal growth. However, as with all studies involving complex multiple risk factors and causes, both measured and unmeasured factors may be influencing the results. There are many known socioeconomic factors or complications of pregnancy that can often occur in the same people, and how these interact deserves future attention.

Where did the story come from?

Professor David Rich and colleagues from the School of Public Health in Piscataway, New Jersey, US carried out this research. The study was funded by grants from the Foundation of the University of Medicine and Dentistry of New Jersey and the Center for Environmental Exposures and Disease.

The study was published in the peer-reviewed Journal of Epidemiology and Community Health.

What kind of scientific study was this?

This was a cohort study comparing birth weights of children born to mothers with different levels of pollution exposure in New Jersey in the US.

The researchers collected data on babies born in New Jersey between 1999 and 2003. They did not use data on twins or other multiple births and only looked at data on babies identified as white, African–American or Hispanic.

From the birth certificate and notes, the researchers collected a range of data on the mother, such as age, race or ethnicity, marital status, education level, smoking, drug and alcohol use during pregnancy. They got more information about the pregnancy from the hospital discharge records, particularly about complications known to affect fetal growth. They estimated the duration of the pregnancy based on the date of the last known menstrual period and clinical judgment (rather than an ultrasound scan).

Air pollution is regularly measured by the New Jersey Department of Environmental Protection, and information about exposure during pregnancy for each mother in her home locality was retrieved from the Environmental Protection Agency website.

The researchers used two main measures of air pollution: nitrogen dioxide (NO2)  levels and a ‘PM 2.5 measurement’ of the amount of solid particles and liquid droplets found in the air that are 2.5 micrometers or smaller in size (soot).

Nitrogen dioxide was measured continuously at 11 stations by the agency and the values for the first, second and third phases (trimesters) of pregnancy were recorded. These measurements were assigned to each mother and baby based on the birth monitor closest to the maternal residence at birth. They excluded all births where the mother did not have a monitoring station within 10km.

The researchers used accepted cut-offs to define babies that were very small for their gestational age (VSGA) or small for their gestational age (SGA). They then used complex statistical tests to determine the relationship between exposure to pollution at three points in the pregnancy, other known risk factors and the chance of having a small or very small baby.

When looking at the effects of air pollution, the researchers took into account available information about the other risk factors such as mother’s age, smoking, drug and alcohol use during pregnancy and other socio-economic factors.

What were the results of the study? 

The researchers found that in the PM 2.5 measurement higher air particulate matter levels during the first and third trimesters significantly increased the risk of small for their gestational age (SGA) baby.

They also found an increased risk of very small for gestational age (VSGA) babies associated with first, second and third trimester nitrogen dioxide concentrations.

What interpretations did the researchers draw from these results?

The researchers conclude that their findings suggest that ambient air pollution may affect
fetal growth. They also suggest that pregnancy complications may increase susceptibility to these effects in late pregnancy.

What does the NHS Knowledge Service make of this study? 

The conclusions of the researchers seem valid and the size of this study adds to the confidence in the suggestion that air pollution, perhaps from traffic emissions, during early and late pregnancy may affect fetal growth.
There are a few points to note about this study:

• It is not clear exactly how air pollution might act to restrict fetal growth and it is possible that other aspects of the women’s lives may have influenced the chance of having a small baby.
• It is plausible, as the researchers say, that air pollution might alter cell activity or cut the amount of oxygen and nutrients a baby receives while in the womb but this will need separate evaluation.
• Studies looking at outcomes where multiple risk factors can affect the outcome are particularly prone to confounding. The researchers have tried hard to take these into account by adjusting for smoking and social-economic background. However, other factors that are linked to small babies, such as diet or maternal height could still be having an effect.
• Only 25% of births had both data on all the risk factors of interest and a residence less than 10km from a monitoring station, making this an urban study. This may limit the application of the results to more suburban or rural areas.
• Ideally, an ultrasound confirmed gestational age would have added reliability to the estimate of duration of pregnancy. Babies can also be small if they are born early.

While this is an important study into the effects of pollution on the chance of having a small baby, there are many additional socioeconomic and obstetric risk factors to take into account when putting this risk into context.