According to the center for disease control, which keeps track of birth information, late summer has been a popular month to have babies. This has been the case for quite some time although we can most recently see it reflected as part of the 2012 National Vital Statistics reports. In 2012, August had the most U.S. births with 361,114 of all the births (approximately 9.1%). July had the second most births at 347,542. February had the least amount, not surprisingly as it has the least amount of days, at 304,505.
However if we look at birth rate (births per 1,000 total population), August is still the highest at 13.6 with September is second at 13.2 and July at 13.1. At the other end of the spectrum the months with the lowest birth rate are January at 11.9 and April at 11.9.
The reason? It has been theorized that people spend more time indoors during the winter months which causes more interactions. About nine months after that, a baby is born.
These are just numbers for the United States, birth frequencies vary across the world based on a variety of factors.