nandini must b some weeks less than tanya as she was in coma whn tanya conceived her baby with karan. n by time tanya knew she was pregnant, she must have been having a foetus of 2-4weeks. and if nandini came out of coma after 3yrs, n til she got back home, she must be pregnant by at least 4-6weeks less than tanya, unless nandini deliver a premature baby.