Purpose: Synthetic oxytocin for labor augmentation during birth has been linked to negative neurodevelopment effects in children. We examined whether maternal labor augmentation was associated with lower cognitive ability in young adulthoods. Patients and methods: We identified 330,107 individuals (96.6% were men), with noninduced labor and with a cognitive ability test score, the Børge Priens Prøve (BPP) score, from draft board examinations in 1995–2015 (mean age, 18.8 years). Information on maternal labor augmentation was ascertained from the Danish Medical Birth Register, and we calculated mean differences in the BPP score according to maternal labor augmentation. We repeated our analyses in a sub-sample of siblings to control for unmeasured familial confounding. Results: Maternal labor augmentation was not associated with any noticeable decline in cognitive ability. However, the difference in the mean BPP score for exposure to maternal labor augmentation varied according to maternal parity, as the mean difference in BPP scores increased with increasing parity, in nulliparous: mean difference=–0.14 (95% CI=–0.23 to –0.04); in maternal parity 4+: mean difference=–1.21 (95% CI=–2.905 to –0.37). The sibling analysis showed little influence of shared familial factors on the association. Conclusion: The mean BPP was slightly lower among labor augmented compared to nonaug-mented and with an increasing difference with increasing parity. However, the differences were small and could not be considered of any clinical relevance. Furthermore, the sibling analyses suggested little confounding by familial factors.