Adverse intrauterine exposures such as maternal smoking and alcohol consumption are known to exert negative impacts on offspring health. However, it is hotly contested whether these exposures have long-term effects that persist across generations. In this study, we performed Mendelian randomization (MR) analysis across three generations in the Norwegian HUNT study to examine if grandparental smoking and alcohol consumption causally influenced grandchildren’s birthweight. Grandparental exposures were proxied by an unweighted genetic risk score (GRS) constructed from genetic variants associated with either number of alcoholic drinks per week, smoking initiation, or number of cigarettes smoked per day (CPD). Our design leveraged all the advantages of the MR method with its robustness against confounding and reverse causality, plus the opportunity to conduct negative control comparisons using grandfathers. More than 20,000 genotyped grandparental-parental duos and trios with grandchild’s birthweight were included in the analysis. Although grandmaternal CPD and drinks per week were observationally associated with decreased grandchild’s birthweight, MR analyses showed no strong evidence for a causal effect of grandparental alcohol consumption or smoking on grandchild’s birthweight when adjusting for the other grandparent GRS, parental GRS, and grandchild’s gestational age, birth year and sex. Additionally, for benchmark analysis, we used 6,006 grandparental-parental trios with parental birthweight to test for the presence of a causal association across two generations. Grandmaternal and grandpaternal alcohol consumption and CPD showed no evidence of a causal effect on parents' birthweight. However, grandmaternal GRS for smoking initiation was, as expected, significantly associated with parental birthweight. In conclusion, there was little evidence of transgenerational effects of smoking and alcohol consumption across three generations, although we cannot rule out small effects. Transgenerational effects of intrauterine smoking and alcohol consumption on birthweight, if they truly exist, are likely to be small.