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The Vanishing Twin: Pathologic Confirmation of an Ultrasonographic Phenomenon

Sulak, M.D., L., Dodson, M.D., M.

Although the phenomenon of the “vanishing twin” has been noted repeatedly through the use of ultrasound, no confirmatory histologic evidence has been presented previously. This has raised questions concerning the validity of the vanishing twin syndrome. A triplet intrauterine pregnancy was diagnosed ultrasonographically four weeks after in vitro fertilization, but only a single fetus and placenta were delivered at term. Careful examination of the placenta revealed at term. Careful examination of the placenta revealed histologic evidence of the vanished twin. This evidence consisted of a chorion-lined sac containing amorphous material, surrounded by degenerated chorionic villi juxtaposed against a normal amniochorionice membrane. (Obstet Gynecol 68:811, 1986)

Landy et al reviewed nine sonographic studies that documented a vanishing twin frequency of between 53 and 78% in multiple pregnancies diagnosed sonographically during the first trimester.

Presented here is pathologic evidence of a vanished twin, and criteria for the diagnosis of similar cases is suggested.

Levi reported that 71% of twin gestation diagnosed sonographically before the tenth week of were indeed singletons when delivered.

Robinson and Caines observed 30 women diagnosed sonographically as having twin gestation in the first trimester. Fourteen patients ultimately delivered twins, for a disappearance rate of 16 in 30 (53.3%).

Loss of one twin during early gestation has a much higher incidence (53-78%) than the reported abortion rate of 10-15% for a singleton pregnancy or, by comparison, an expected 20-30% chance of at least one or both twins being aborted, if such events were independent. This suggests that local influences or competition between twins may result in an exceptionally high vanished twin rate during early pregnancy. In triplets and quadruplets, the effect of early fetal competition between twins may be even more intense. In reporting 300 early pregnancies evaluated by means of ultrasound noted 21 twins, one triplet, and one quadruplet. Only three pairs of twins survived to term (85.6% loss of the second twin), whereas the triplet pregnancy evolved into a normal twin gestation, and the quadruplet pregnancy reported here also evolved into a singleton gestation, and the quadruplet pregnancy evolved into a singleton gestation. The triplet pregnancy reported here also evolved into a pregnancy diagnosed by means of ultrasound that evolved into a twin gestation. These very high resorption rates, which cannot be explained on the basis of the expected abortion rate, again suggest intense fetal competition for space, nutrition, or other factors during early gestation, with frequent loss or resorption of the other twin(s). Another explanation would be an exceptionally high incidence of genetically or embryologically abnormal embryos in multiple gestation, with loss of one twin during the early gestational period.

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