Environmental conditions experienced during foetal and neonatal life influence early development with potential long-term consequences for later life events. In many parts of the world, the external environment varies with the season. This includes factors such as photoperiod, climatic factors, nutrition or prevalence of infectious agents. Birth season is one surrogate for the environmental conditions experienced at conception, during pregnancy, around birth and during early postnatal life. The seasonal variation of environmental factors may thus influence early pre- and postnatal developmental processes differently in individuals born at different times of the year, causing respective downstream effects on later life events. Accordingly there is accumulating evidence that effects of birth season may impact a variety of physical, physiological and psychological parameters in humans. This review covers a survey of birth season effects on reproductive characteristics in women together with a discussion of possible causes, focussing on environmental factors that might play a central role during development, crucial periods of early development that might be influenced by these environmental factors, and potential underlying physiological mechanisms.