Twins are prone to developmental delay due to prematurity and low birthweight. However it is unknown if twinning is an independent risk factor for developmental delay. The objective of this study was to compare the attainment of a set of gross motor milestones in a cohort of twins and singletons in The Gambia. Eighty-four pairs of twins and 72 singletons were enrolled at birth and followed up until 18 months of age. The mean age at achieving milestones was higher in twins for each development outcome and the difference between twins and singletons was significant after adjustment for confounders for maintaining head, sitting without support and walking. In twins, we found a highly significant correlation within pairs for most milestones. When monozygotic and dizygotic twins were compared, a significant heritability was observed for crawling, sitting, standing and walking, with over 90% of population variance observed due to genetic factors rather than environmental factors. There was little evidence for a genetic contribution towards very early milestones. In conclusion, our data suggest that twinning is an independent risk factor for developmental delay in early life in The Gambia, and that genetic factors contribute strongly to certain motor development outcomes.