While his wife was in the hospital giving birth, he had sex with his unsuspecting mother-in-law who was helping with housework. (Yumi Anno, NACR-930)

While my wife was in the hospital awaiting delivery, her mother came to help with housework. Seeing his mother-in-law doing chores in extremely revealing clothing, Junichi's pent-up sexual desire, accumulated since his wife's pregnancy, was aroused. His mother-in-law was initially reluctant, but when she remembered she was a woman, she told him this would be the last time and willingly accepted Junichi's advances...