Gestational age and risk of intellectual disability: a population-based cohort study.

Objective: To examine the association between gestational age at birth and risk of clinically diagnosed intellectual disability (ID) week by week to provide a detailed description of ID risk across the entire range of gestational ages and by severity of ID.

Methods: All individuals born alive in Sweden 1974-2017 were prospectively followed up from birth until 2017 using national registers. The HRs for ID according to weekly gestational age and gestational age categories were determined using Cox models. Sibling analyses were conducted to adjust for familial confounding.

Results: The study included 3 572 845 live births. During the follow-up, 26 596 ID cases were registered. The adjusted weekly estimates showed a gradual increase in risk of ID from week 40 to week 24 (adjusted HR=1.80 (1.74 to 1.87), aHR=3.93 (3.73 to 4.13), aHR=7.53 (6.95 to 8.16), aHR=21.58 (18.62 to 25.00)) and from week 41 onwards (aHR=1.26 (1.19 to 1.32)), with statistically significantly higher risks across the range of gestational age compared with infants born at week 40. The associations were consistent in mild, moderate and severe/profound ID but most prominent for severe/profound ID.

Conclusion: The risk of ID increased weekly as the date of delivery moved away from 40 weeks, both preterm and post-term. The results remained robust after detailed adjustment for confounding, including familial confounding.