Limited data exist about the ideal timing of developmental screening for young children entering foster care, and current best practice recommends screening by 1 month into care to prioritize resources for evaluation. Therefore, we aimed to: (1) compare detection rates for potential developmental delay (DD) at foster care entry before and after implementation of a developmental screen and (2) examine accuracy of developmental screening when performed at entry and 1 month into care. Charts of 124 children <6 years evaluated for an initial foster care health assessment (IFCHA) were reviewed to determine baseline detection rates for potential DD. The Parents’ Evaluation of Developmental Status (PEDS) screening tool was then prospectively administered to 167 children <6 years during their IFCHA to determine detection rates. One month following the IFCHA, caregivers were re-contacted, and the screen was re-administered. Accuracy of the initial PEDS screen was compared to the 1 month PEDS screen by calculation of sensitivity and specificity. At baseline, potential DD was detected in 34 % of children at the IFCHA compared to 46 % after implementation of the PEDS (P = 0.041). Compared to the 1 month screen, the early screen had a sensitivity of 75 % and specificity of 88 %. Use of a developmental screening tool at foster care entry increased detection of potential DD, and the results remained consistent with screening 1 month later. These results support use of a developmental screen for children in foster care and suggest that screening be performed as early as possible to expedite necessary evaluations and referrals.