Several factors could explain this phenomenon. First, hot temperatures in the summer tend to reduce sperm production. Less sperm in the man equals less chance of pregnancy. While this can impact all women, research has shown that women of lower socioeconomic status are more affected than others. If a woman can hardly pay the bills, how can she spring for reproductive help like in vitro fertilization? As a result, these lower educated, lower paid women as less likely to give birth in the spring and summer months. Another explanation is what researchers are calling the "prom babies" effect, accounting for the higher number of winter births to teenage mothers. Nine months after all that Dirty Dancing and it's snowing outside.