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Study Question: Does preconceptional exposure to oil-based iodinated contrast media during hysterosalpingography (HSG) impact children's neurodevelopment compared with exposure to water-based alternatives?, Summary Answer: Our study found no large-sized effects for neurodevelopment in children with preconceptional exposure to oil-based iodinated contrast media during HSG compared with water-based alternatives., What Is Known Already: HSG is widely used as a diagnostic tool in the female fertility work-up. Tubal flushing with oil-based iodinated contrast has been shown to enhance fertility outcomes in couples with unexplained infertility, increasing the chances of pregnancy and live birth compared with water-based alternatives. However, oil-based contrast contains higher doses of iodine and has a longer half-life, and concerns exist that iodinated contrast media can affect women's iodine status and cause temporary (sub)clinical hypothyroidism in mothers and/or foetuses. Considering that thyroid hormones are vital to embryonal and foetal brain development, oil-based contrast media use could increase the risk of impaired neurodevelopment in children conceived shortly after HSG. Here we examine neurodevelopmental outcomes in school-aged children conceived after HSG., Study Design, Size, Duration: This is a long-term follow-up of the H2Oil trial in which oil-based or water-based contrast was used during HSG (Netherlands; 2012-2014; NTR3270). Of 369 children born <6 months after HSG in the study, we contacted the mothers of 140 children who gave consent to be contacted for follow-up. The follow-up study took place from January to July 2022 (NCT05168228)., Participants/materials, Settings, Methods: The study included 69 children aged 6-9 years who were conceived after HSG with oil-based (n = 42) or water-based contrast (n = 27). The assessments targeted intelligence (Wechsler Intelligence Scale for Children), neurocognitive outcomes (computerized neurocognitive tests), behavioural functioning (parent and teacher questionnaires), and academic performance. Linear regression models, adjusted for age, sex, and parental educational attainment were employed to compare groups., Main Results and the Role of Chance: School-aged children born to mothers after oil-based contrast HSG did not significantly differ from children born to mothers after water-based contrast HSG, in regards to intelligence, neurocognitive functioning, behavioural functioning, or academic performance, with the exception of better performance for visuomotor integration functions in children exposed to oil-based contrast preconception. After exploratory correction for multiple comparisons, none of the group differences was statistically significant., Limitations, Reasons for Caution: The small sample size of this follow-up study limited statistical power. This study provides evidence for the absence of large-sized differences between preconceptional exposure to the two contrast media types but does not rule out more subtle effects on neurodevelopment compared to naturally conceived children without preconceptional exposure to HSG., Wider Implications of the Findings: This study contributes to our knowledge about the long-term effects of different types of iodinated contrast media used in fertility work-up, indicating that choosing oil-based over water-based iodinated contrast media is unlikely to have major effect on the long-term neurodevelopmental outcomes of children conceived shortly after HSG. However, further research should focus on the overall safety of iodine exposure during HSG, comparing children conceived after HSG to those conceived naturally as both types of contrast contain high amounts of iodine., Study Funding/competing Interest(s): The original H2Oil randomized controlled trial was an investigator-initiated study that was funded by the two academic hospitals now merged into the Amsterdam University Medical Centre. The current follow-up study (Neuro-H2Oil) is funded through a research grant awarded to the authors by the Amsterdam Reproduction & Development (AR&D) research institute. S.K. is funded by a AMC MD/PhD Scholarship from the Amsterdam UMC. S.K. reports holding voluntary roles in the civil society organizations Universities Allied for Essential Medicines and People's Health Movement. V.M. reports receiving travel and speaker fees as well as research grants from Guerbet, Merck and Ferring. K.D. reports receiving travel and speaker fees as well as research grants from Guerbet. BWM is supported by a NHMRC Investigator grant (GNT1176437) and reports consultancy, travel support and research funding from Merck, consultancy for Organon and Norgine, and holding stock from ObsEva. The other authors report no conflict of interest., Trial Registration Number: NCT05168228., (© The Author(s) 2024. Published by Oxford University Press on behalf of European Society of Human Reproduction and Embryology.)