Why Should Duphaston Be Taken After Getting Pregnant?

I m diagnosed with PCOS, i already had a baby 3 yrs ago. I never had any miss carriage.as i want to concieve again and my cycles are not regular so my doctor suggested me to take glucophage along with duphaston for concieving. even she said that after concieving i wil have to continue these both medicines. now i want to ask that previously i have never misscariaged so whats the need of taking duphaston after getting pregnant ??