Pregnancy Timeline: Calculating Due Date & Understanding Test Results

Question
Well I'm about as confused as ever. Let me just explain everything in detail. My last 2 periods were June 4 & June 29. They are NEVER early. My 2nd one was lighter than usual, but definitely more than spotting. I had sex on July 12 & 16. I has a very faint positive pregnancy test on July 26th, as well as 3 more in that weekend. I went to the Dr August 1st & had another more noticeable positive test. Calculating with my last period as well as when I had sex, I would be 8 weeks today due April 6. I have been spotting brown blood since this past Friday. No cramping, just brown blood & brown goo. I had a intravaginal sonogram today, could see the baby's heartbeat (but not hear it) she said it had a heart rate of 112 which I though was low but she said it's fine. She did the measurements & said I am 6 wks 0 days. Which would make my conception date July 28...2 days AFTER a positive pregnancy test?! When I pointed that out, she had no explanation of how that is possible. So obviously it would seem to me that I'm 8 weeks like I thought, but the baby is just not the size it should be. And when I look at sonogram pictures online, it definitely looks like 6 week sonograms, not 8 week. Does that mean the baby will probably miscarry since the size isn't matching? And wouldn't it make more since for my Dr to think logically & know I'm actually 8 weeks, just with a 6 week sized fetus? I'm so confused and worried & would appreciate any help. Thanks so much in advance!

Answer
It seems to me that you do have a smaller-than-average baby but you are 8 weeks and you conceived around the 19th or 20th (so from the July 16th encounter). I would tell your physician, though she's most likely going off of averages and sometimes babies grow at a slower rate than others. The later due date may actually help your baby more as she seems to be a bit of a slow starter growth-wise. =)  If you smoke, this may have something to do with it as babies of smokers tend to be smaller.

Please let me know if I can help you further or with anything else.

Best wishes,
Helen =)