Background: The neurodevelopmental effects of fetal exposure to most antiseizure medications are unclear. We aimed to investigate the effects of fetal exposure to commonly used antiseizure medications on neuropsychological outcomes at age 3 years., Methods: The Maternal Outcomes and Neurodevelopmental Effects of Antiepileptic Drugs (MONEAD) study is a prospective, observational, multicentre cohort study at 20 specialty epilepsy centres in the USA. We have investigated pregnancy outcomes in women (aged 14-45 years) with and without epilepsy who were enrolled during pregnancy (≤20 weeks' gestational age), and their children. The primary outcome for children at age 3 years was a blindly assessed Verbal Index score, which was calculated by averaging scores on the Naming Vocabulary and Verbal Comprehension subtests of Differential Ability Scales-II, Expressive Communication and Auditory Comprehension subscales of Preschool Language Scale-5, and Peabody Picture Vocabulary Test-4. Children of women with and without epilepsy were compared, and the associations of medication exposures to outcomes in exposed children were assessed. The MONEAD study is registered with ClinicalTrials.gov, NCT0730170, and is ongoing., Findings: Between Dec 19, 2012, and Jan 13, 2016, 456 pregnant women (351 with epilepsy and 105 without epilepsy) were enrolled into the study. 345 children were born to women with epilepsy and 106 children were born to women without epilepsy. Verbal Index scores at age 3 years did not differ for children of women with epilepsy (n=284; adjusted least-square mean 102·7, 95% CI 101·4 to 103·9) versus those without epilepsy (n=87; 102·3, 99·8 to 104·7). Significant risk factors for reduced Verbal Index scores included maternal intelligence quotient, maternal education, post-birth anxiety, gestational age at enrolment, child's sex, and child's ethnicity. For Verbal Index scores, antiseizure medication exposure effects were not seen for maximum third trimester blood concentrations (n=258; adjusted parameter estimate -2·9, 95% CI -6·7 to 1·0). However, in secondary analyses, exposure-dependent effects were present on multiple cognitive measures, which varied by medication., Interpretation: We found no difference in neurodevelopmental outcomes between children with fetal exposure to newer antiseizure medications compared with unexposed children. However, some exposure-dependent antiseizure medication effects were seen in secondary analyses. The adverse effects of maternal post-birth anxiety emphasise the importance of screening mothers during pregnancy and postpartum and implementing interventions. Additional studies are needed to clarify the exposure-dependent effects., Funding: National Institutes of Health, National Institute of Neurological Disorders and Stroke, and National Institute of Child Health and Development., Competing Interests: Declaration of interests KJM has received research support from the National Institutes of Health, Eisai and Sunovion Pharmaceuticals; the Epilepsy Study Consortium pays KJM's university for his research consultant time related to Eisai, GW Pharmaceuticals, NeuroPace, Novartis, Supernus, Upsher-Smith Laboratories, UCB Pharma, and Vivus Pharmaceuticals. DWL reports active research support from National Institute of Neurological Diseases and Stroke (NINDS) and National Institute of Mental Health; he is a Consultant for NeuroPace, receives book royalties from Oxford University Press and editorial stipend from Epilepsia and Neuropsychology Review; he is an employee of Emory University and derives clinical income from neuropsychological patient evaluations. AKB reports receiving grant support, paid to her institution, from Supernus Pharmaceuticals and Veloxis Pharmaceuticals, holding patent US9770407B2 on parenteral carbamazepine formulation, licensed to Lundbeck, and patent EP12150783A on novel parenteral carbamazepine formulations, licensed to Lundbeck. PEV reports speaker honoraria from Neurodiem, Stony Brook University, and Physicians’ Education Resource. EEG received research support from Sage Pharmaceuticals, Sunovion Pharmaceuticals and Xenon Pharmaceuticals, and Eisai/Stanford, and received speaker from Greenwich Pharmaceuticals and UCB Pharma, travel funds from UCB Pharma, and honoraria from Neurology Week. JC received research support from NINDS (The Maternal Outcomes and Neurodevelopmental Effects of Antiepileptic Drug [MONEAD] study) and from GW Pharmaceuticals. MS reports receiving advisory board consulting fees for Aquestive. JAF receives salary support from the Epilepsy Foundation and for consulting work or attending Scientific Advisory Boards on behalf of the Epilepsy Study Consortium for Aeonian/Aeovian, Alterity Therapeutics Limited, Anavex, Arkin Holdings, Angelini Pharma, Arvelle Therapeutics, Athenen Therapeutics/Carnot Pharma, Autifony Therapeutics Limited, Baergic Bio, Biogen, Biohaven Pharmaceuticals, BioMarin Pharmaceutical, BioXcel Therapeutics, Bloom Science , BridgeBio Pharma, Camp4 Therapeutics Corporation, Cerebral Therapeutics, Cerevel, Clinical Education Alliance, Coda Biotherapeutics, Corlieve Therapeutics, Crossject, Eisai, Eliem Therapeutics, Encoded Therapeutics, Engage Therapeutics, Engrail, Epalex, Epihunter, Epiminder, Epitel, Equilibre BioPharmaceuticals, Greenwich Biosciences, Grin Therapeutics, GW Pharma, Janssen Pharmaceutica, Jazz Pharmaceuticals, Knopp Biosciences, Lipocine, LivaNova, Longboard Pharmaceuticals, Lundbeck, Marinus, Mend Neuroscience, Merck, NeuCyte, Neumirna Therapeutics, Neurocrine, Neuroelectrics USA Corporation, Neuronetics, Neuropace, NxGen Medicine, Ono Pharmaceutical, Otsuka Pharmaceutical Development, Ovid Therapeutics, Paladin Labs, Passage Bio, Pfizer, Praxis, PureTech LTY, Rafa Laboratories, SK Life Sciences, Sofinnova, Stoke, Supernus, Synergia Medical, Takeda, UCB, Ventus Therapeutics, Xenon, Xeris, Zogenix, and Zynerba. She has also received research support from the Epilepsy Study Consortium (funded by Andrews Foundation, Eisai, Engage, Lundbeck, Pfizer, SK Life Science, Sunovion, UCB, Vogelstein Foundation) Epilepsy Study Consortium/Epilepsy Foundation (funded by UCB), GW/FACES and NINDS. She is on the editorial board of The Lancet Neurology and Neurology Today. She is Chief Medical/Innovation Officer for the Epilepsy Foundation. She has received travel reimbursement related to research, advisory meetings, or presentation of results at scientific meetings from the Epilepsy Study Consortium, the Epilepsy Foundation, Angelini Pharma, Clinical Education Alliance, NeuCyte, Neurocrine, Praxis, and Xenon. AMP reports funding from National Institutes of Health (NIH), royalties from UptoDate, and travel reimbursement for American Academy of Neurology and American Board of Psychiatry and Neurology activities. PBP reports grants from NIH, personal fees from NIH for grant reviews, personal fees from American Epilepsy Society as speaking honoraria, personal fees from AAN as speaking honoraria, personal fees from Medical Schools for speaking honoraria and travel, and personal fees from UpToDate for Royalties outside the submitted work. All other authors declare no competing interests., (Copyright © 2023 Elsevier Ltd. All rights reserved.)