In SS wedding, Alaap did a big tamasha and was repeating and stressing on Suki's raaz with confidence. He also mentioned Suki's visits to Ashram and so on.
On the day of Suki's pagphera, milind heard S & P taking about hiding some raaz.
Ritwik's B'day party, Aysha said everything including Ashram's recirds and all.
All these things being said in all tamasha's can not be forgotten ( at least I can't forget things being said especially comments that insulted me). If Milind use his intelligence and put all this things together, at least he will realize that S & P are together in hiding whatever raaz. If Milind even believe that P is deceiving him, he won't doubt Suki at all. In that case, Milind can use common sense and think that if even if there is any baby raaz connected to P , Why would Suki hide it form her brother?