In this review, the influence of seasonal variation on undernutrition during the first 1000 days of life of life in rural South Asia is conceptualized using a modified framework developed under the “Tackling the Agriculture and Nutrition Disconnect in India” project. Evidence for the existence and extent of seasonality is summarized from 14 studies reporting on six proximal determinants of undernutrition. A limited number of studies examine seasonal variation in risk factors for this age group. All available studies, however, report a compelling finding of significant seasonal variation for at least one determinant of undernutrition. Research to clarify mechanisms for potentially adverse effects of seasonal variation on health and nutritional status during the first 1000 days of life is needed.