Journal of Ultrasound in Medicine, Vol 14, Issue 2 117-121, Copyright © 1995 by American Institute of Ultrasound in Medicine
The empty amnion: a sign of early pregnancy failure
K. M. McKenna, V. A. Feldstein, R. B. Goldstein and R. A. Filly
Department of Radiology, University of California, San Francisco 94143-0628, USA.
Between 6.5 to 10 weeks of gestation, the length of the amniotic cavity is
similar to that of the embryo. It follows that by the time an amniotic sac
is detectable sonographically, an embryo of equal length should also be
visualized. Retrospective review of case records at our institution
revealed 15 patients in whom the amnion was visualized in the absence of an
embryonic pole during first trimester sonography (endovaginal and
transvesical). Indications for sonographic examination included gestational
age estimation, discrepant size and dates, or vaginal bleeding. The mean
sac diameter for the 15 gestations ranged from 14 to 36 mm, corresponding
to gestational ages of 6.1 to 9.5 weeks. Ages based on the last menstrual
period ranged from 6.1 to 11 weeks. A yolk sac was identified in all cases
in addition to the amniotic sac, but neither an embryo nor cardiac
pulsations were observed. In 12 of the 15 cases the size of the gestational
sac was greater than 16 mm, such that the absence of an embryo also met an
accepted criterion for a failed pregnancy. Follow-up in all cases confirmed
early pregnancy failure. In this series the demonstration of an "empty
amnion" (visualization of an amnion but no identifiable embryonic pole) was
always associated with pregnancy loss. The "empty amnion" sign is helpful
as an additional finding confirming early pregnancy failure.
