NoirRoseHeart:
Is there a different drama called this because the one I'm thinking of I don't remember it having one. The child in it is the FL's brother. The ML misunderstands all by himself that it is his child when it is not, from my memory.
It has been a few years since I’ve seen this drama but I remember the kid plot the same way as NoirRoseHeart does. FL’s mother got pregnant around the same time they had a one night stand which explained the ML’s confusion when he saw the FL with a kid (her younger brother) of that certain age. FL was never pregnant.