BACKGROUND: Hyperemesis gravidarum is characterized by severe nausea, vomiting, and potentially undernutrition during pregnancy. Despite affecting up to 3% of all pregnancies, little is known about the impact of hyperemesis gravidarum on the cognitive development of the child. OBJECTIVES: We aimed to compare the school performance of offspring exposed to HG during pregnancy with their non-exposed counterparts and siblings. STUDY DESIGN: We conducted a population-based cohort study including all liveborn children, born in Denmark between 1 RESULTS: Of the exposed offspring, 29.3% had low school performance compared to 25.7% of the non-exposed group. After adjustment for confounders (birth year, maternal age, multiple pregnancies, parity, maternal education, maternal smoking, and sex), we found that exposed offspring had increased odds of low school performance (odds ratio 1.19 (95% confidence interval 1.14 to 1.24). Furthermore, the exposed group had a lower mean grade point average of 6.55 compared with 6.78 in the non-exposed group, yielding an adjusted mean difference of -0.18 (-0.22 to -0.14). However, within-sibling analyses, comparing exposed offspring with their unexposed siblings (same mother), revealed no increased odds of low school performance (adjusted odds ratio 0.98 (0.92 to 1.04) or difference in mean GPA (-0.02 (-0.08 to 0.05)). CONCLUSION: We found a small association between in-utero exposure to hyperemesis gravidarum and low school performance. However, no difference in school performance was observed between exposed and unexposed siblings, indicating that the association could be attributed to genetics or other familial factors rather than hyperemesis gravidarum.