Hans Zinsser, MD, while an officer in the US Army Medical Corps in World War I
“Obstetrics is not the pleasantest of medical occupations, although it pays well and is one of the things that the young physician with any kind of practice can count on as a as financial backlog. Yet it takes a great deal of time and means a lot of night work. While the statement may not be statistically correct, it does seem to the medical man as though the large majority of all babies were born at night. An observant medical student in my class once asked one of our instructors about this. 'Dr. V., why is it that most children are born at night?' Dr. V,. who was something of a wag, replied: 'Well my boy, that’s simple. It takes just nine months.'"
From Zinsser, Hans. As I Remember Him: The Biography of R.S. Gloucester, Mass: Peter Smith. 1970.