To examine the timing of reaching developmental milestones in children born post-term.
The Danish National Birth Cohort: children born between 1997 and 2003.
Data were obtained from a cohort of 92 892 pregnancies participating in the first pregnancy interview. All singletons born in gestational weeks 39-45 were identified. The study was then restricted to children who participated in an interview at the age of approximately 18 months and had information on at least one developmental milestone. We excluded children of mothers with chronic diseases from the final analysis. The remaining study population constituted of 43 915 singletons (27 503 born at term; 16 412 born post-term).
Logistic regression was used to calculate odds ratios of late achievement of these developmental milestones, adjusted for potential confounding factors.
Achieving developmental milestones at the time of interview or at a certain age.
More children born post-term achieved the assessed developmental milestones compared with children born at term (39-40 weeks). A test for trend for gestational ages 39, 40, and 41 weeks also showed a positive trend at achieving developmental milestones with gestational age at birth in nine out of 14 milestone items.
Children born post-term appear to reach the main developmental milestones at an earlier age than children born at term. The association could also result from bias related to a longer time between conception and interviewing, misclassification of end points, or selection bias.