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Air pollution linked to premature births

Pregnant women who are exposed to high levels of air pollution in the early part of pregnancy are more likely to have their baby early, a study suggests. But the overall risk remains low and we’re not sure that the findings apply to all women.

BMJ Group News

What do we know already?


Doctors refer to babies born before the 37th week of pregnancy as preterm babies. Some preterm babies can have health problems both as infants and during later life. This is more likely if a baby is born before 32 weeks. Babies born before 26 weeks are very underdeveloped and often struggle to survive.

There’s an emerging school of thought that pregnant women who are exposed to air pollution are more likely to have preterm babies. This is based on studies that have observed pregnant women in different parts of the world. But we’re still a long way from being sure this link is genuine.

Now Swedish researchers have studied 135,000 pregnant women who gave birth in hospitals in Stockholm between 1998 and 2006. They recorded details of the mothers’ health, their pregnancy, and their children’s health. Specifically, they looked at measurements of two pollutants in the air (a chemical called ozone and vehicle exhausts) during the first 12 weeks a woman was pregnant, and looked to see if the levels of these air pollutants affected her risk of having a baby early.

They also looked at air pollution and a link to having complications of a preterm birth, such as having a small baby, or having a condition called pre- eclampsia. They also explored whether women with asthma were more likely to have these kinds of problems.

What does the new study say?

Around 4 women in every 100 had a preterm baby, and around 3 in 100 women had pre-eclampsia.

Women who were exposed to higher levels of ozone during the first 12 weeks of their pregnancy were more likely to have a preterm baby. After taking into account things we know can affect a woman’s risk of having a preterm baby - like her health and whether she was a smoker or had previous children - the researchers calculated the risk of preterm birth increased by four percent for every unit increase in ozone levels in the air. (A unit of ozone is 10 micrograms per cubic metre.)

The researchers stated the link between ozone levels and an increased risk of preterm delivery is definitive, but their figures show it’s not possible to rule out the link being due to chance.

They also found a similar link between increases in the level of ozone and a raised risk of pre-eclampsia. But again, we can’t rule out this being due to chance.

The researchers say their figures suggest women with asthma are 25 percent more likely to have a preterm baby and 10 percent more likely to develop pre-eclampsia. But it’s difficult to be sure how they arrived at these figures.

There was no link between vehicle exhaust fumes and preterm births.

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