Journal of Ultrasound in Medicine, Vol 17, Issue 4 199-203, Copyright © 1998 by American Institute of Ultrasound in Medicine
"Appearing twin": undercounting of multiple gestations on early first trimester sonograms
P. M. Doubilet and C. B. Benson
Department of Radiology, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.
An important role of first trimester sonography is to determine whether a
pregnancy is a singleton, twin, or higher order multiple gestation. We
assessed how frequently sonography at 5.0-5.9 weeks undercounts multiple
gestations. We identified all pregnancies at our institution since 1988 in
which (1) an initial sonogram obtained at 5.0-5.9 weeks demonstrated at
least a singleton intrauterine pregnancy and (2) a subsequent sonogram at
6.0 weeks or beyond demonstrated a living multiple gestation. Twenty-four
(11%) of 213 dichorionic twin gestations were initially undercounted as
singletons, as were six (86%) of seven monochorionic twin gestations. Among
105 higher order multiples, 17 (16%) were undercounted initially. All but
one of the undercounted cases were scanned transvaginally. Undercounting
occurred in both natural and assisted conceptions, and it occurred more
frequently on sonograms obtained at 5.0-5.4 weeks than at 5.5-5.9 weeks (P
= 0.02, Fisher's exact test). Prognosis for undercounted multiple
gestations was similar to that of correctly counted ones with respect to
several measures of pregnancy outcome, including the likelihood that all
fetuses would be delivered liveborn, gestational age at birth, and birth
weight (P > 0.20, all comparisons). In conclusion, transvaginal
sonography at 5.0-5.9 weeks frequently undercounts multiple gestations.
Initially undercounted multiple gestations and those correctly counted have
similar pregnancy outcomes.
