Question Title: early ultrasound--larger gestational sac than embryo

I am approx. 7 weeks pregnant by my LMP, but about 5-6 weeks by my calculations. I had an ultrasound yesterday, and my gestational sac measured me at 7 weeks pregnant, but the embryo was about 5 weeks along. There was no cardiac activity (too soon), and I see my dr. in two days. I am a wreck though, having miscarried two months ago, and sure don't want to repeat that! Is it normal to have a sac larger than the embryo? Why would they measure different gestational ages? I don't remember seeing a yolk sac either, but I didn't get to see the pictures very well, and the technician didn't explain much to me. Any info would be greatly appreciated.

Dear Emilie:
Pregnancy is counted from the last menstrual period. Most ladies do not know the day of ovulation/conception (that is changing with ovulation detection kit availability). If you do know the date of conception, that allows adjustment because the assumption behind counting from LMP is that a lady ovulated on cycle day 14.
Ultrasound measurements are taken by "eyeballing" the crown (top of head) and rump (at this early stage there are no formed legs). At 5 weeks from LMP the CR length is 2-5 mm. At 7 weeks, the length is 6-11 mm. The calipers are about 1 mm and the error in measurement is about 4 days. The sac is substantially larger than the fetus at this early stage and the larger size reduces the error within the measurement. Thus, this measurement error is accounting for the dating differences.

Keywords: ultrasound, early pregnancy

This information isprovided for education purposes only and is not a medical consultation. If you have specific questions, please contact your physician.

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