Ambient particulate matter and preterm birth or birth weight: a review of the literature.

To review epidemiologic evidence on maternal exposure to particulate matter and adverse pregnancy outcomes, we performed a MEDLINE search of the literature up to June 2009. We considered all original studies published in English including information on total suspended particles (TSP), respirable (PM(10)) or fine (PM(2.5)) particles and the risk of preterm birth, low birth weight (LBW) or very low birth weight (VLBW) and small for gestational age (SGA). We identified a total of 30 papers, including 13 with information on preterm birth, 17 on LBW or VLBW, and 4 on SGA. Eight studies on preterm birth, 11 studies on LBW/VLBW and two studies on SGA reported some increased risk (by about 10-20%) in relation to exposure to PM; no meaningful associations was found in the remaining studies. However, even in studies reporting some excess risk, this was inconsistent across exposure levels and pregnancy periods. Epidemiologic studies on maternal exposure to PM during pregnancy thus do not provide convincing evidence of an association with the risk of preterm birth and LBW/VLBW and SGA. The excess risks, if any, are small, and it is unclear whether they are causal, due to misclassification of the exposure or some sources of bias/residual confounding.