China radically reduced the air pollution in preparation of the Olympics in 2008 in Beijing. The authors of the publication Differences in Birth Weight Associated with the 2008 Beijing Olympic Air Pollution Reduction: Results from a Natural Experiment [Rich2015a] studied its affect on the weights of newborns. They evaluated how specific months of pregnancy with less air pollution correlate to the birth weights, compared to pregnancies during the same dates in 2007 or 2009. The records were created and maintained by the Beijing Obstetrics and Gynecology Hospital and for the study they just included singleton live births of infants who were 28 completed weeks of gestation or greater in one of four adjacent Beijing districts at the time of birth, which is a sample size of 140,298.
And indeed, they found a correlation:
Babies with their 8th month of pregnancy during the 2008 Olympics were, on average, 23g larger
Interestingly, there was apparently no correlation for months 1-7 of pregnancy!?