What is colour doppler scan.. N in which week it has to be done? I hv scheduled an app win my 34th week

a Doppler scan is like  sonography, to assess babies health. it measures blood flow in some vessels of the baby and mother, to see if baby is receiving adequate nutrients and oxygen.
everyone may not need a Doppler. so do it if and when your gynec suggests.

Dr. Payal M which week doc did not day anything... But once she had informed that doppler is done in 34 weeks..

you will need her referral note to get the scan done. the timing will also be suggested by her. so you should consult her for it.