I would also assume the doc is right.... that is also a BIG gap in time and would make TONS of difference in the pregnany from March to May or June.... I am 30 weeks and conceived near the end of June! I would definitely say the Dr is correct on this one!
since you had a period in march,april and may.then im guessing your doctor is right you dont have periods whilst pregnant,you may have bleeding but not period,periods shed the lining of your uterus,so not possible,i agree with your doctor.
Have you had an ultrasound yet? That should tell you approx. how far along you are. My menstral cycles are ALL OVER THE place, so I had an ultrasound at about 7 weeks to determine length of time..
I would ask your Dr. for an ultrasound. That is the best way to tell in early pregnancy. Good luck and keep us posted and congrats on the pregnancy!