Well in season one episode three or four Cersei tells Kat sitting at Bran’s bedside that she had Robert’s son and it died of fever. Little bird with no feathers is how she described it. Did not abort that one. She said it died, was taken away and she never even visited the crypt. Cersei thinks much of her blood doubt she would destroy her own kind, she might send it away and hide it. Have not read the books. And Cersei is a first class liar and manipulator. Poor Nedd is too honorable for his own good but still a man and Cersei is very beautiful.
The books are full of “unreliable narrators”. Cersei in particular is a liar, and you can’t trust what she says.
Yes, she was lying to Cat in order to be able to fake sympathy with her.
My point was, Ned would not have had a child with Cersei, from his book POV chapters, he despised her.