Spoilers below in the entire answer:
Although K spends most of the movie figuring out that he is the lost son of a replicant and then believing this to be true, it's eventually revealed that he cannot be the son of Deckard and Rachael... Because their child was a girl, not a boy. This is revealed by Freysa after she has saved K.
The identity of the child was obfuscated in several ways. K is made to believe that Rachael and Deckard had twins, a girl and a boy, but quickly deduces that something is amiss about the records because their DNA on file is identical. Twins of different sex can't have the same DNA because they're fraternal twins. So there was ever only one child, not twins. This is obviously corroborated by everyone in the know; there's only one child on the photo, Freysa only mentions one child, and Deckard only mentions one child. These are more than "indications".
In the records, the girl is listed as deceased, while the boy lived. In fact, the records were forged; there was no boy, he was made up to send anyone looking on a wild goose chase. This is corroborated by Freysa.
K started the movie believing he was made, with implanted memories. He then thought he was born, and that the memories were real. In fact, as revealed by the end of the movie, he was made and the memories were implants – but they were real, implanted memories from the actual daughter of Deckard and Rachael.