I had an ultrasound on June & the doctor said my due date was on October 18, 2012. Then I had another ultrasound on September and the doctor said my due date is October 10, 2012, but if I count 38 weeks back from those due dates that would mean my baby was conceived before my period. Could I have been pregnant during my period?? The first day of my last period was January 17, 2012 & I know it was definitely my period because it was the whole 7 days.

1. While rare it is possible to be pregnant and have a last period.

2. more common the fetus is smaller than normal or larger than normal and they just use average fetus size as a estimate for age of fetus.

These dates are guesses at best, and are normally good, give or take a couple weeks.

Thus a good example of why we say that

38 weeks is just an average.
Date of conception is just an estimate at mid cycle, and yes, you can have a period, or even more than 1.
Determining age on ultrasound is an estimate too.
Unless you only had sex once in all of January, you will never know when you conceived.