I was born at 1:23PM on a Monday, and there's nothing remarkable about that — by US standards, anyway. Births in the country are daytime affairs, reports the CDC in a new study of birth certificate data from 2013. If a child is born during the weekday, chances are, they were born during the morning and midday hours. And the reason for that is far from biological. The fact is that more babies are born during the day because that's when medical professionals schedule cesareans — a delivery method that makes up one-third of US births.

If births were equally timed throughout the day, about 4.2 percent of babies born in a single day would be born each hour. But because the number of births that take place each day tend to happen between 8AM and 5:59PM — prime working hours for hospital staff — the percentage of births that happen each hour is much higher during that window of time. "The highest percentages of births occurred during the 8:00AM and noon hours," with 6.3 and 6 percent of births, respectively, the authors of the CDC report note. Less than 3 percent of babies are born each hour between midnight and 6:59AM, however. And weekends show a more evenly distributed pattern of deliveries.

The method of delivery also has an impact. In 2013, the US saw about 3.9 million births. Of those, more than 98 percent were hospital deliveries. These can be broken down into four categories: non-induced vaginal births, induced vaginal births, unplanned cesareans, and planned cesareans. Vaginal births that aren’t initiated by drugs or surgical means, for instance, are more likely than cesareans and induced vaginal births to occur in the evening and early morning. In contrast, planned cesarean deliveries are much more concentrated during the day than unplanned cesarean deliveries. Overall, induced vaginal births rise in the morning, peak in the mid-afternoon, and then decline starting at 6PM.

Out-of-hospital deliveries follow a different pattern, however. They are more likely to happen in the early morning hours — a time when hospitals see the lowest percentages of births. Then, they slowly decrease until about 12PM, when there’s a short uptick in births. After that, births don’t increase again until about 3PM.

The difference in timing of hospital births and out-of-hospital deliveries suggests that medical interventions — interventions that have increased in frequency over the past few decades — are playing a big role in determining the timing of children's' birth. But the study only took into account 90 percent of all US births, so it may not be representative of the entire country. That will change soon, however; time-of-birth data will be available for the entire country starting in 2016.