The three graphs above show that women who give birth in winter months (blue dots) tend to be younger, less educated, and less likely to be married compared to mothers who give birth during the summer months (green dots). The data displayed some trends for the time period shown (1996-2001), but the summer/winter discrepancy remained surprisingly constant.
These data, reported by economists Kasey Buckles and Daniel Hungerman at the University of Notre Dame, may offer an explanation for the observation that, compared to "summer babies," those born in winter months tend to do more poorly in school, are less healthy, earn less, and have shorter lifespans.
The mechanism behind these relationships, alternative explanations, and a long comment thread are available at the primary link.
Link, via Salon.