Pregnancy Timeline & Due Date Calculation: Seeking Clarity

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
Dear Heather,

I calculate also (based on LMP and intercourse dates you provide) that you would have been 8 weeks on 8/24/08. The fact that you had a sonogram indicating a fetus which measured much smaller may be significant and prompt your doctor to follow up with hCG levels done 48 hours apart. In early pregnancy we would expect the hCG level to double every 48-72 hours. When that level does not rise (double) appropriately, it does indicate a threatened miscarriage situation.

If that hCG test would come back doubled and within normal limits for this point in the pregnancy, then the expected course of action would simply be to wait and see what happens and watch those levels. The fact that there was a heartbeat was encouraging, but again the 48-hour hCG level is a very important indicator as to the health of the pregnancy and I know very few medical professionals who will make a determination as to the health of a pregnancy based on the ultrasound information alone.

I hope this has helped you and answered your question. I wish you well.

Brenda