We all know that Karan visited Nandni , not daily but atleast after 2-3 days. Nandni told Karan that when she was free (guess after 4-6 years), she visited SN and saw Tanya and Bhoomi all happy and that's why she did not came back.
If Karan still visited Nandni before she was released and her disappearance, she must have given birth to her child.
Even when she came to SN, and saw Tanya and Bhoomi happy, she did not have her child with her.