ObjectiveTo examine the correlation between birth season and early childhood development.BackgroundAlmost all previous studies that examine the effect of birth season on early childhood development were conducted in developed countries with a limited sample size. The present study was conducted in poor, rural areas of western China, a developing region with a continental monsoon climate.MethodWe administered a hemoglobin test to 650 infants (52% boys), aged 8–10 months, using a Hemocue Hb 201+ finger prick system, and assessed the cognitive and psychomotor development of sample infants using Bayley Scales of Infant Development.ResultsInfants born in winter have higher Hb concentrations (t = 3.63, p < 0.001) compared to infants born in summer. Similarly, cognitive development scores (t = 5.17, p < 0.001) and psychomotor development scores (t = 10.60, p < 0.001) were significantly higher among winter-born infants.ConclusionThe findings point to the involvement of birth season in early childhood development and suggest that aspects of the environment shape the experiences that contribute to early childhood development. Policy suggestions such as providing infants with ample opportunities for movement and stimulation during the cold season are discussed.