Pronounced and persistent seasonal patterns in fertility are observed in virtually all human populations. This paper presents evidence on these seasonal patterns. We note that the most pronounced seasonal patterns are in the southern United States, where births decline substantially in April and May, and in northern Europe, where births increase substantially in March and April. Although seasonal variations in fertility were more pronounced in earlier agricultural populations, we show that seasonality has increased in this century in some high income, low fertility populations such as Sweden. We use data on monthly temperature to analyze the potential role of temperature in explaining seasonal patterns. We find strong evidence that summer heat plays an important role in explaining the July-August trough in conceptions in the southern United States. We find little evidence, however, that temperature plays any role in explaining the pronounced June-July peak in conceptions in Sweden. Temperature also appears to be relatively unimportant in several other populations with substantial seasonal variations in births, suggesting that other factors play an important role in birth seasonality.