From a Seneca County, New York newspaper in April 1863:
A Young Corporal.
The Rochester Union says that a private letter from an officer in the 20th Reg. N.Y.S.V., to a friend in that city states that a Corporal in the regiment last week gave birth to an infant. It was not until quite recently that the sex of the Corporal was discovered. Her husband is a Sergeant in the regiment. She enlisted as a private and was promoted for good conduct.
Pregnancy would kind of put a dent in any 1860s version of “Don’t Ask, Don’t Tell”
The only official policy was against female recruits, but as Karen Abbott writes in an interesting article at Wonders & Marvels, “Male soldiers who knew or suspected there was a woman in the ranks were usually impressed by her patriotism and bravery, and chose to keep her secret.” Sometimes women outed themselves by behaving characteristically for women of 150 years ago. For example,
Two women serving with the 95th Illinois Infantry were outed when an officer threw apples to them. They were dressed in full military uniform, but instinctively made a grab for the hem of their nonexistent aprons in order to catch the fruit.
According to Ms. Abbott six women served while pregnant during the Civil War.
So far I haven’t seen any other mention of a female corporal in the 20th.