What it says up top. Either Hydra pimped out the Winter Soldier to the Red Room as part of a supersoldier breeding program, or Nat is the result of a female agent's dalliance with the Soldier, and Hydra sold the baby. Nat doesn't know her origins; Bucky never knew what became of his baby. They find out the truth together in the present day from some recovered files.

I am aaall about the awkwardness and pain and weird family feelings that would ensue. How would Natasha, who is so kind and grounded but still wrestles with her identity and history, take this as a new part of her story? How would she feel knowing this traumatized sort-of-friend is her biological father, and would she worry she had become just another reminder of his trauma? On Bucky's part, would that be true? Or is there sweetness to be found here as well?

+ even if we're going the breeding program route, Bucky's semen wasn't extracted medically, or at least not exclusively. I'd prefer for Natasha to be the result of a sexual rape

+ for that matter, the more horrific and traumatizing that rape was, the better. If there are details or terrible implications about what a nightmare Bucky experienced in the files, available for Nat's reading pleasure, that would also be great