• Air pollution
• Pregnancy Outcome
• Stillbirth

Stillbirth, or late fetal death, is defined by the WHO as a child born with no signs of life and a birth weight of 1000 g or more, a gestational age of 28 weeks or more, or a length of 35 cm or more.1 It is estimated that 2.6 million children worldwide were stillborn at 28 weeks or more in 2015, primarily in low-income and middle-income countries.2 Half of the stillbirths occur during labour and congenital abnormalities account for less than 10% of stillbirths.2 Stillbirth is an important, yet poorly understood, adverse outcomes of pregnancy. The large geographical variation suggests that the majority of stillbirths are preventable.

In this issue of Occupational and Environmental Medicine, Siddika et al3 reviewed 13 original epidemiological studies that addressed the question of whether maternal exposure to ambient air pollution triggers stillbirth. They conducted a meta-analysis to summarise the existing evidence, and reported evidence of an association between exposure to ambient air pollution and elevated risk for stillbirth. A 4 μg/m³ increase in exposure to particulate matter (PM) with a diameter of less than 2.5 μm (PM_2.5) during pregnancy was associated with an increased risk of stillbirth (summary OR 1.02, 95% CI 1.00 to 1.05; 17 033 cases).3 Differences in design and pollutants assessed precluded all 13 studies to be meta-analysed, and thus their analysis was based on only two or three large register-based studies from the USA or Asia. Exposure to other routinely monitoring air pollutants, including nitrogen dioxide …