So in Fool's Quest Kettricken mentions she knows that Verity borrowed Fitz body to provide a Farseer heir, because she's not sure if Fitz will come back from his journey.
She doesn't do that before he sets out with the company to the outislands to slay IceFyre previously, nor when Fitz is in critical danger, best examples to my mind is when he's critically injured after killing Laudwin's horse and is near death's bed. Furthermore she blames herself as the possible source of Dutiful's Wit in the first book of the Fool trilogy, which wouldn't seem fair or logical if she knew that Fitz is essentially Dutiful's father.
So why would she not make mention of this earlier, or if she only recently learned of it, how?
"We both fell silent for a time. Then she spoke in a husky whisper. "Thank you for my son." I lifted my eyes from the map. I just looked at her. "I've known for years. How it was done." I didn't ask how she knew. Starling had possibly told her. Perhaps Verity himself. "Your body. Verity's will." "I wasn't there, Kettricken. I spent that night inhabiting Verity's body." "He's Verity's son. I know" And we left it there, and I was not certain if I felt better for her knowing and letting me know she knew or if I felt even odder about it. I only asked her, "Are you telling me this because you don't think I'll come back?" She met my gaze. "I think you left when you lost Bee, " ..