Measuring small @ First Ultrasound

[Malenkaya]

Last Tuesday, 7/21, I had my first ultrasound for this pregnancy, and I would be due in March.  The day I went in, I had calculated being 7w4d.  However, the ultrasound measured 6w2d.  We did hear the heartbeat immediately, it was 107.  Which they said was fine for 6w2d.

However, that calculates the conception date as 6/21.  I got a positive pregnancy test on 6/25.  My HCG was 226 on 6/26 and progesterone was 14.4.  On 6/29 my HCG was 1054.  It seems strange my HCG would be so high so soon and it also feels doubtful I could have gotten a positive pregnancy test 4 days after conception.

I go back tomorrow for a second ultrasound.  The doctor didn't really seem concerned, just said they don't normally do ultrasounds so early so they wanted to repeat in a week to check growth rate and make sure the heart rate also increases (they want to hear between 130-170).

Anyone have any similar experiences and how did it turn out?  I know sometimes they say you can be off because you ovulated late or implanted late, but I have had consistent 32 day cycles with ovulation on day 18.  Ovulating later than day 18 seems unlikely, not to mention getting a positive (very positive, dark line) on a pregnancy test only 4 days after conception?  The HCG is also high for that stage.  Before the u/s we were actually thinking twins.  There are several sets of fraternal twins on my mom's side of the family, I am nearly 37 and this is my 4th pregnancy - all things said to increase the odds for twins.  I've read of cases of a twin being missed on an early ultrasound, and although I'm not expecting twins at this point, I haven't ruled it out.  I am mainly concerned about the discrepancy in the dates that I can't find an reliable explanation for.

[MommyTinasaur]

Wow, all I got at my first ultrasound was a dark room, quiet tech and some pictures. She didn't tell me anything about the baby or how far along she thought I was. Didn't listen for the heart rate... nothing really. This was about 5-6 weeks ago.. I'm about 19 weeks now and no one has checked my babies heart rate at all =[

[shenac]

This happened to me on 8-4-09. I went to the doctor for my first ultrasound, at 9w4d and she said it looked like I was only 6 weeks along.
I know I am right about my dates, the doctor said there was a 90% chance I miscarried, but they did blood work, and my hcg levels were over 75000. I went back today for more blood work, I will know more tomorrow. Waiting *****.

[Elaine Brown, MD]

Hi!
Congratulations!
Keep in mind that ultrasound measurements are only estimates.  Based upon the technique of the technician and the  type of machine used, etc.  there can be a variation of 3-7 days in a very early ultrasound.  The farther along in pregnancy you get, the higher that discrepancy gets, so that in the third trimester, an ultrasound estimate of your gestational age can be off by up to 3 weeks.
I think repeating the ultrasound is a fine idea.  It may change your due date again. In fact each ultrasound you get will probably give you a different estimate.  If there has been appropriate growth, you know that things are going well. Once a heart rate is seen on a scan the odds that the pregnancy will continue are very good!
It will also give you another chance to rule in/out twins.

I hope this is helpful!
Good luck!
Dr B