Dear KMS: Alas, I am not a perinatologist so this will be a more general answer. As you likely know the gestational sac appears at about 4-5 weeks followed by the appearance of the yolk sac. That you have still not seen an embryo or cardiac motion does suggest a blighted ovum. For a good overview on the ultrasound assessment of pregnancy check this link:
http://www.brooksidepress.org/Products/Military_OBGYN/Ultrasound/1st_trimester_ultrasound_scannin.htmAfter reviewing MULTIPLE studies at the National Library of Medicine site, this seemed to best address discordant yolk sac size:
Radiology. 1992 Apr;183(1):115-8.
Yolk sac diameter and shape at endovaginal US: predictors of pregnancy outcome in the first trimester.
Lindsay DJ, Lovett IS, Lyons EA, Levi CS, Zheng XH, Holt SC, Dashefsky SM.
Department of Radiology, Health Sciences Centre, Winnipeg, Man, Canada.
Four hundred eighty-six consecutive women who underwent endovaginal sonography when their fetuses were less than 10 weeks menstrual age (MA) were evaluated to establish the normal size and shape of the secondary yolk sac (YS) and to assess the value of YS measurement in predicting pregnancy outcome in the first trimester. A YS diameter more than two standard deviations (SDs) above the mean when compared with the mean gestational sac diameter allowed prediction of an abnormal pregnancy outcome with a sensitivity of 15.6%, a specificity of 97.4%, and a positive predictive value of 60.0%. A YS diameter more than two SDs below the mean allowed prediction of an abnormal outcome with a sensitivity of 15.6%, a specificity of 95.3%, and a positive predictive value of 44.4%. No pregnancy with a normal outcome had a YS diameter of greater than 5.6 mm at less than 10 weeks MA. In six patients, the YS diameter was greater than 5.6 mm. All six had an abnormal outcome. Of seven patients with abnormal YS shape at initial sonography, three had abnormal YS shape at follow-up examinations. All three had an abnormal outcome.
It sounds like your OB is being very thorough. I hope the next ultrasound will help to clarify your situation.
In Support,
Jane