Developmental disability rates may vary by the season or month in which children are born. However, the seasonal mechanisms, such as climate, underlying these variations are unclear. Previous studies focused on high-latitude regions, leaving this climate hypothesis only partially tested. We analyzed rates of developmental disabilities in 3- to 5-year-old children from a tropical climate--South Florida--from 2004 to 2015 (N = 79,047) using a time-series analysis to identify peaks in diagnoses based on birth months. Children with speech impairments were more likely to be born in March, April, and September, a novel finding. Children with developmental delays were more likely to be born in September and October, consistent with studies in colder regions. In contrast to prior studies, there were no effects of birth month for language impairment or autism spectrum disorder. We highlight potential prenatal mechanisms that may contribute to the seasonal variations in some developmental disabilities.