Talk to Gynecologist on Pregnancy

I was on duphaston from 15 to 25 day of my menstrual cycle and got positive result after 5 days of stopping duphaston....but doctor does not see any gestational sac on ultrasound and i do urine test on different strips but still positive and i am again on duphaston from last 8 days to prevent early miscarriage but last day doctr cannot see any sac... My last period was on 29 December and my average cycle is almost 29 days