That is the question Milind asked Prachi on November 7th (11/7) and again on January 7th (1/8). Prachi's answers to the same question has been diametrically opposite. While on 11/7 she claimed to be Ritwik's mom, on 1/8 she claims she is not. The only thing consistent between then and now is that at both times she refused to name the true mother.
What about Milind' reaction to her responses? While on 11/7 the guy had atleast 98 per cent faith in her (I am deducting 2 per cent because by then Ayesha had successfully sowed some seeds of doubts in him) and would have believed anything she had said (maybe with a little bit of emphasis to make up for the 2 per cent loss of faith), on 1/8 the guy seems to have had only 2 per cent faith in her (and that too out of sheer desperation of not wanting to lose her). Her refusal to come clean with him atleast now just wiped out even that 2 per cent faith. The half truth instead had the opposite effect, with Milind swearing that he will get the divorce at all costs.
If half truth is what she wanted to come out with the best time to have done so was at Ritwik's birthday party when Ayesha made her first accusation. At that point of time Milind's faith in her was so strong that he told her very clearly that he trusted her more than himself. Had she told him then that while she was not Ritwik's mother as accused by Ayesha, there is a secret behind his parentage which she was not in a position to tell him, Milind would have accepted her. He may have even defended her when Ayesha made her accusations on 11/7. But no, she waits for more than two months to dish out the half truth when Milind has already assumed the worst of her.
This is the price that both Mi and Chi are paying for her folly.