Researchers pooled 63 meta-analyses of observational studies to see which factors were tied to ADHD. Forty covered environmental risk or protective factors and 23 covered blood-based markers. The people studied were children, since the review treated ADHD as a childhood condition.
Five factors had the strongest ("convincing") evidence of a link. These were the mother being obese before pregnancy (OR 1.63), childhood eczema (OR 1.31), high blood pressure disorders in pregnancy (OR 1.29), pre-eclampsia (OR 1.28), and the mother using acetaminophen in pregnancy (RR 1.25). Four more had "highly suggestive" evidence, including the mother smoking in pregnancy (OR 1.6), childhood asthma (OR 1.51), being overweight before pregnancy, and lower vitamin D in the blood. Breastfeeding was the only factor tied to lower odds of ADHD (OR 0.7), and that evidence was weak.
The authors stress these are associations, not proof of cause, because the studies were observational. When they looked at sibling studies, the links for maternal smoking, obesity, and overweight were not replicated, which points to shared family or genetic factors. The blood-marker studies were small, disagreed with each other, and were rated lower in quality.
Source: Kim J, Kim J, Lee J, et al.. Environmental risk factors, protective factors, and peripheral biomarkers for ADHD: an umbrella review. The Lancet Psychiatry (2020). doi:10.1016/S2215-0366(20)30312-6