Air pollution exposure in early pregnancy and adverse pregnancy outcomes

BMJ Open 2013;3:e001955 doi:10.1136/bmjopen-2012-001955

• Occupational and environmental medicine

Air pollution exposure in early pregnancy and adverse pregnancy outcomes: a register-based cohort study

1. David Olsson1, 
2. Ingrid Mogren2, 
3. Bertil Forsberg1

1. ¹Department of Public Health and Clinical Medicine, Occupational and Environmental Medicine, Umeå University, Umeå, Sweden
2. ²Department of Clinical Science, Obstetrics and Gynaecology, Umeå University, Umeå, Sweden

1. Correspondence toDavid Olsson; david.olsson{at}envmed.umu.se

• Received 2 October 2012
• Revised 3 December 2012
• Accepted 3 January 2013
• Published 5 February 2013

Abstract

Objectives Our aim was to study the possible associations between exposure to elevated levels of air pollution, ozone (O₃) and vehicle exhaust (NO_x), during early gestation, and adverse pregnancy outcomes such as pre-eclampsia, preterm birth and small for gestational age.

Design Prospective register-based cohort study.

Setting The Swedish Medical Birth Register includes data on all deliveries during 1998 to 2006 in Greater Stockholm, Sweden. The national Patient Register and the Prescribed Drug Register were used to collect information on maternal asthma.

Participants All singleton pregnancies, conceived at the earliest in August 1997 and at the latest in February 2006, were included, n=120 755.

Outcome measures We studied preterm birth, small for gestational age and pre-eclampsia.

Results 4.4% of pregnancies resulted in a preterm birth. The prevalence of pre-eclampsia was 2.7%. We observed an association between first trimester O₃ and preterm birth (OR 1.04, 95% CI 1.01 to 1.08) as well as an association with pre-eclampsia (OR 1.04, 95% CI 1.01 to 1.08), per 10 µg/m³ increase in O₃. We observed no association between first trimester NO_xand adverse pregnancy outcomes. No associations were observed between any of the air pollutants and small for gestational age.

Conclusions Increased levels of O₃ during the first trimester increased the risk of pre-eclampsia and preterm birth. Air pollutants did not exhibit any effects on fetal growth restriction. We estimated 1 in every 20 cases of pre-eclampsia to be associated with O₃exposure.

ARTICLE SUMMARY

Article focus

• Does early gestation air pollution exposure, in particular ozone (O₃), affect the risk of adverse birth outcomes (ie, pre-eclampsia, preterm birth and small for gestational age)?

Key messages

• This large European study adds to the evidence that preterm birth may be caused by O₃exposure, and that the effect may be greater among asthmatic mothers.
• This is one of the first studies to show an association between O₃ and pre-eclampsia.
• Health impact assessment of O₃ exposure should include also effects on pregnancy outcomes.

Strengths and limitations of this study

• Temporal fluctuations in air pollution exposure are unlikely correlated to individual level risk factors, however still accounted for in order to improve the model performance.
• This study uses valid estimates of temporal variation in O₃ exposure, adjusts for seasonality, but does not take into account any spatial variation in the traffic pollutants.
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