Once Rehaan finds out that the baby is his, he will rightly so want it back and by all means Naina should give the baby back. If her hesitation is that he is planning to kill the baby then she should tell him what Ahana said to her and perhaps she can say she will keep the baby until he can prove he wants the baby and is a good father.
What I don't agree with and I am sure the show will go this way is that Raina are keeping the baby saying it's theirs. Firstly it is not theirs to keep and secondly Naina took the money for Raghav to become rich and now if she keeps the baby too that just makes her look selfish.
She very well knew what she was doing and for them to show that a surrogate mother takes the child and considers it as their own isn't sending a good message to those who want to go for this option. They would always have the fear that the surrogate mother will take their child.
Ekta has used this track in her other shows and I hope she does justice here. The child should be given rightfully to Rehaan.