Objectives: Early identification of developmental vulnerability is vital. This study aimed to estimate the prevalence of moderate or high developmental risk on the Parents' Evaluation of Developmental Status (PEDS) at 6-month, 12-month and 18-month well-child checks; identify associated risk factors; and examine documentation of the PEDS at well-child checks. Design, participants: A prospective birth cohort of 2025 children with 50% of those approached agreeing to participate. Demographic data were obtained via questionnaires and linked electronic medical records. Telephone interviews were conducted with parents to collect PEDS data. Primary and secondary outcomes: Multiple logistic regression analyses identified risk factors for moderate or high developmental risk on the PEDS. A Cumulative Risk Index examined the impact of multiple risk factors on developmental risk and documentation of the PEDS at the well-child checks. Results: Of the original cohort, 792 (39%) had 6-month, 649 (32%) had 12-month and 565 (28%) had 18-month PEDS data. Parental concerns indicating moderate or high developmental risk on the PEDS were 27% (95% CI 24 to 30) at 6 months, 27% (95% CI 24 to 30) at 12 months and 33% (95% CI 29 to 37) at 18 months. Factors associated with moderate or high developmental risk were perinatal risk (OR 12 months: 1.7 (95% CI 1.1 to 2.7)); maternal Middle Eastern or Asian nationality (OR 6 months: 1.6 (95% CI 1.1 to 2.4)), (OR 12 months: 1.7 (95% CI 1.1 to 2.7)); and household disadvantage (OR 6 months: 1.5 (95% CI 1.0 to 2.2). As the number of risk factors increased the odds increased for high or moderate developmental risk and no documentation of the PEDS at well-child checks. Conclusions: Children with multiple risk factors are more likely to have parental concerns indicating.