"The results showed that either prenatal exposure to MSDP [maternal smoking during pregnancy] or smoking cessation during first trimester was significantly associated with childhood ADHD [attention-deficit hyperactivity disorder] after adjusting for parental psychiatric history and social socioeconomic status."
"With our meta-analysis, we provide evidence for an association between maternal smoking and offspring ADHD but do not solve the causality issues concerning potential confounding by other risk factors."
Those quotes were taken from the findings reported by Tianyu Dong and colleagues  and Lan Huang and colleagues  respectively, both applying the 'top of the methodological tree' analysis that is a meta-analysis to the collected data looking at whether maternal smoking during pregnancy might impact on offspring risk of a diagnosis of ADHD. Their combined answer: very possibly but...
Having previously covered the non-association between maternal smoking during pregnancy and offspring autism before on this blog (see here), the recent results from Dong and Huang highlight how tobacco smoking during pregnancy is still something to be avoided for the sake of the child(ren). The fact that Huang et al also observed something of a dose-response relationship: "The risk of ADHD was greater for children whose mothers were heavy smokers (OR: 1.75; 95% CI: 1.51-2.02) than for those mothers were light smokers (OR: 1.54; 95% CI: 1.40-1.70)" kinda adds to the strength of their argument for an *association* between smoking during pregnancy and adverse offspring developmental outcome(s).
It's true that even with such meta-analytical prowess, it's still a little difficult to say conclusively that 'smoking causes offspring ADHD'. I'm also minded to state how "different assessment tools of ADHD and a lack of objective biological measures for maternal smoking" were picked up by the authors as other reasons to be cautious about their [collected] findings. I will also direct you to other research published not-so-long-ago that did not find such a strong connection between maternal smoking during pregnancy and offspring ADHD  (or at least "indicating that the association between maternal smoking during pregnancy and offspring ADHD is not directly causal, but confounded by unmeasured factors.")
Still, even if maternal smoking during pregnancy represents just one of a number of factors potentially impacting on offspring risk of ADHD, it is worth reducing that risk by nudging mums-to-be (and perhaps even dads-to-be) to quit smoking for the sake of their children. This on top of the myriad of other reasons that parental tobacco smoking during conception, pregnancy and offspring childhood is something that really needs to be stamped out once and for all...
 Dong T. et al. Prenatal Exposure to Maternal Smoking during Pregnancy and Attention-deficit/hyperactivity Disorder in Offspring: A Meta-analysis. Reprod Toxicol. 2017 Dec 30. pii: S0890-6238(17)30599-3.
 Huang L. et al. Maternal Smoking and Attention-Deficit/Hyperactivity Disorder in Offspring: A Meta-analysis. Pediatrics. 2018 Jan;141(1). pii: e20172465.
 Gustavson K. et al. Smoking in Pregnancy and Child ADHD. Pediatrics. 2017 Feb;139(2). pii: e20162509.