At my 31weeks 5days scan,parameters were:bpd 76mm, fl57mm, ac261mm, hc288mm. placenta site:anterior, grade: 2, praevia:nil. cx is 3.5cm long,os closed.is everything normal?also at my 34weeks scan yesterday,doctor said i have to do a doppler scan at 36 weeks.what could be the reason he opted for doppler scan?