Genetics do most of the work, but not all of it. Research suggests that somewhere between 10% and 40% of the variation in ADHD risk comes from non-genetic factors. (Thapar et al., 2017) 

A handful of prenatal and early-life factors are associated with a higher chance of ADHD: premature birth, low birth weight, exposure to tobacco smoke or alcohol before birth, and exposure to toxins such as lead. These are worth knowing about. 

But here is the important caveat, and it is one a lot of articles get wrong. Association is not the same as cause. When researchers use genetically informed studies - comparing siblings, or comparing mothers’ and fathers’ exposures - much of the apparent link weakens. It turns out the same genes that raise ADHD risk also make some of these exposures more likely in the first place. So these are best understood as risk markers, not proven causes, and the evidence does not support any single one of them “causing” ADHD on its own. (Thapar et al., 2017)