Serial ultrasound examinations have demonstrated that one of two gestational sacs in a twin pregnancy may often disappear. When it disappears at an early stage of gestation, the pregnancy may advance without any disturbance and the cotwin can be delivered well developed and lively. When the intrauterine death occurs in the second trimester, the dead fetus usually results in a fetus papyraceus and the cotwin continues to be alive near term. However, when death occurs in the last trimester, the viable twin may be spontaneously delivered soon and be premature. In some cases of late fetal death, the dead fetus may induce intravascular thromboses in many organs of the surviving cotwin, so that the living infant may develop cerebral palsy later after birth.

In general, it has not been recognized that many twin or multiple gestations are lost in utero early in pregnancy. Until the advent of ultrasound, the ability to document early human fetal loss in multiple gestation was difficult. However, recent reports of serial ultrasound examinations of pregnant women have documented the “disappearance” of at least one of two gestational rings. Furthermore, the number of twins observed at delivery was significantly less than the number of twin conceptions originally identified by ultrasound during the first trimester. These observations led to the concept of the “vanishing twin” [8,10,13,15,17,20,22,35,41].

In order to obtain reference and personal data on this subject, we reviewed the literature and corresponded with members of the International Society for Twin Studies and obstetricians affiliated with Northwestern University Medical School, Chicago, Illinois. The nine studies that have documented this phenomenon cite “disappearance” rates ranging from 0% to 78%, depending on patient population and timing of ultrasonography [8,10,13,15,17,20,22,35,41]. Several explanations are offered: physiological mechanisms of “disappearance” (resorption or formation of a blighted ovum or fetus papyraceus), artifactual error, incomplete scanning technique, and poor quality ultrasound equipment. The only complication thus far associated with “disappearance” of a fetus is slight vaginal bleeding.