Journal of Ultrasound in Medicine, Vol 14, Issue 6 415-419, Copyright © 1995 by American Institute of Ultrasound in Medicine
Using the number of yolk sacs to determine amnionicity in early first trimester monochorionic twins
B. Bromley and B. Benacerraf
Department of Obstetrics and Gynecology, Massachusetts General Hospital, Harvard Medical School, Boston, USA.
The purpose of this study was to evaluate the relationship between the
number of yolk sacs and amnionicity in monochorionic twin pregnancies
scanned early in the first trimester. We retrospectively reviewed images of
all monochorionic twins scanned between 6 and 9.5 weeks' gestation and with
pathologic or sonographic confirmation of chorionicity-amnionicity. Each
film was reviewed for the number of yolk sacs present, as well as for the
gestational age at which the amniotic membrane was first visualized. Twenty
monochorionic-diamniotic pregnancies and two monochorionic-monoamniotic
pregnancies met the criteria for inclusion in the study. In diamniotic
pregnancies scanned at less than 8 weeks' gestation, only the yolk sacs
were identified; none of the dividing amniotic membranes were detected. Two
yolk sacs were identified in all but one case. In this case, although one
yolk sac was seen at 6 weeks, follow-up scanning at 8 weeks revealed two
yolk sacs. In each of the monochorionic-monoamniotic twin pregnancies, one
yolk sac was seen at 9 weeks and a single amnion encircled both embryos. We
conclude that the sonographic identification of two yolk scas in
monochorionic twins enables us to make the diagnosis of diamniotic twins
early in the first trimester, before the amniotic membrane can be imaged.
The presence of one yolk sac should prompt a follow-up ultrasonogram to
assign amnionicity definitively.
