Actually, we don't know much about the circumstances surrounding the AnuSar kid's kidnapping and what Anu was told by AS.
But, if she spent her entire pregnancy depressed and an emotional mess mourning Kalyani, it's not that unfathomable that she felt disconnected from the baby she was carrying and doesn't remember much about those days.
Sarthak, for obvious reasons, has a right to feel bad about it.
I'm just getting fed up with Kalyani being caught up like this in a situation she did nothing to create.
It's really painful to see her again and again become the victim of Sarthak's anger when she has done nothing to him.
Also, I saw the episode and Malhar didn't mention or remember the date as the day when his cousin-sister passed away, either, so....
Not saying a cousin and a mother are the same thing, just that maybe everyone has moved on and only Sarthak keeps remembering his child in a way that only brings him more pain.