That Mel did not go straight from HS to Nursing school. Also, she was 18 or 19 when we first met Mel. So she would be at least 20 now. Mia is 17 or 18 as she would be a senior in HS. Whether it is a 1 year or 2 year difference, Carly would have to know when she gave birth to this baby.
Also, she knew who she gave the baby too, because she said that to Bo once. That Lawerence made her give the baby to that awful man. It was her punishement.