A possible explanation for the inverse association for PM2.5 could be due to the “air pollution paradox” (Ritz, 2009). It is well known that smaller particles (PM2.5) compared with larger parti- cles are more harmful because of different chemical composition, with more combustion-related sources, and therefore, may increase the proportion of early spontaneous abortions, and selective survival of more viable fetuses (pregnancies without congenital malformations). There is evidence that heavy metals may be bound to fine particles and such exposure may lead to an increased risk of spontaneous abortions (Bellinger, 2005; Rahman et al., 2010; Weselak et al., 2008). Another possible hypothesis, which needs further research, is that chemical mixtures such as polycyclic aromatic hydrocarbons bound to particles, may induce abortions (Wu et al., 2010).