Question: Does tobacco smoking during pregnancy elevate the risk of offspring autism?
Answer: According to the meta-analysis published by Shiming Tang and colleagues  (open-access), probably not.
I have to say that I was interested in the paper from Tang et al. Interested not only because of their findings suggestive that smoking during pregnancy did not seem to elevate the risk of offspring autism, but also that their report seemed to pretty much replicate a previous meta-analysis by Brittany Rosen and colleagues  in both design and findings. I've talked previously about the Rosen paper on this blog (see here).
To quote from Tang: "The random-effects model was used to combine results from individual studies." To quote from Rosen et al: "Using a random-effects model, we found no evidence of an association."
Same with the results - from Tang: "The pooled odds ratio (OR) was 1.02 (95% confidence interval (CI): 0.93–1.13) comparing mothers who smoked during pregnancy with those who did not." Figures that weren't a million miles away from those previously published by Rosen et al - "summary OR 1.02, 95 % CI 0.93–1.12."
Tang do make reference to the Rosen meta-analysis in their text and the fact that they: "only searched two databases which missed several eligible studies" also that they "misunderstood the difference between the period of “prenatal” and “pregnancy”, and they inappropriately included studies that only reported smoking during pregnancy but not specified whether it was during the prenatal period or not." That both papers eventually ended up with 15 studies in their final analyses and pretty much identical results should be noted however.
 Tang S. et al. A Meta-Analysis of Maternal Smoking during Pregnancy and Autism Spectrum Disorder Risk in Offspring. Int. J. Environ. Res. Public Health 2015; 12: 10418-10431.
 Rosen BN. et al. Maternal Smoking and Autism Spectrum Disorder: A Meta-analysis. J Autism Dev Disord. 2015 Jun;45(6):1689-98.
Tang, S., Wang, Y., Gong, X., & Wang, G. (2015). A Meta-Analysis of Maternal Smoking during Pregnancy and Autism Spectrum Disorder Risk in Offspring International Journal of Environmental Research and Public Health, 12 (9), 10418-10431 DOI: 10.3390/ijerph120910418