Later, Moses is almost killed as he begins his journey with his wife and son. Zipporah, Mrs. Moses, responds to the threat on her husband's life with circumcising their son. Probably not my knee-jerk response to the situation.
She is a Midianite. Midian was a child of Keturah and Abraham. Moses was from Jacob's line. I would imagine that circumcision was a well-known procedure in each of their cultures. Given Zippy's response of immediate circ'ing, it seems the ritual was known and perhaps one that had even been discussed in their home, perhaps even recently.