it cant be kasak. i hv the video of prerna's pregnancy during kasak time. vishakha is there..but not pregnant , whereas prerna is almost 8-9 months pregnant. if vishakha got pregnant AFTER that, her child wud still HAVE to be 6 months younger than kasak. and then where did prerna's child from bajaj go? if that child died, then she wud hv been shown mourning for that child, like she mourns for prem1. and if that child got kidnapped, she wud hv definitely searched for him.
besides, now i dont think prerna wud lie to bajaj that his granddaughter is his daughter, for 20 yrs.
Vishakha's child is hidden smwhere. and i think this child will play a major role in the serial from now onwards- as this will be prerna and rishabh's first grandchild.