After graduating, deku left for America. Katsuki and deku did have a one night stand before he left

When deku got to America, and a girl took advantage of him, getting herself pregnant

Deku had a private case and charged the girl getting full custody of the child. The girl was given a restraining order and a 19-year sentence.

[4 years later] The child is now 3 and dekus back in Japan. No one knows about the child

There is a class reunion Katsuki was talking to his friend Kirishima