According to Center for Disease Control and Prevention, there were more births in September of 2010 than any other month that year, followed by August, June, and July, in that order.
That led me to wondering whether that holds true for Washington State, or King County for that matter.
I did the numbers and it turns out March is the highest birth month for King County, followed by August, September and June for 2010. And overall in 2010 for Washington State, August is the highest birth month followed by September, May and July. Data is based on Washington State Department of Health birth data.
You can check out the numbers, in this visualization I created that shows the trends of births by year and month for Washington State counties from 1980 to 2010.