*University of New Mexico School of Medicine, Department of Obstetrics and Gynecology 2211 Lomas Blvd., NE, Albuquerque, NM 87121 Ph: 505-272-6386 Fax: 505-272-6385

Synonyms: "Vanishing Twin Syndromeâ€, vanishing twin.

Definition: First trimester loss of a member of a twin gestation.

Prevalence: The loss of a member of a twin pair occurs with a frequency of 53 to 71% in twins diagnosed prior to ten weeks gestation.

Etiology: After the death of a twin, the water within the twin"s tissues, the amniotic fluid, as well as placental tissue may be resorbed.

Associated anomalies: None known.

Differential diagnosis: Placental tumors such as chorioangioma, placental cysts.

Prognosis: The prognosis for the surviving twin is generally excellent but depends on the etiology of the death of the dead twin.

Recurrence risk: Not known.

Management: Routine surveillance is sufficient, given the generally excellent prognosis.

MESH Twins ICD9 651 CDC 651.032

Introduction

Prior to the use of ultrasound, the diagnosis of the death of a member of a multiple pregnancy, remote from delivery, was made on incidental abdominal radiography or by the pathologist examining the placenta. The death of a twin (or member of a multiple gestation) had been thought to be a relatively rare event with an incidence of about 1:12,000 live births¹. The frequency, gross, and histologic features are all dependent on the time and method of diagnosis, the time of fetal demise, and the diligence of the examiner inspecting the placenta after delivery.

When a twin dies after the embryonic period of gestation, the water within the twin"s tissues, the amniotic fluid, as well as the placental tissue may be resorbed. This results in the flattening of the dead twin from the pressure of the growing twin². Posner and Klein postulated that "the physical character of the dead fetus would appear to depend upon its position in the uterus with reference to the viable twin, the amount of amniotic fluid in each sac, the integrity of the chorion, and the time of death″. The dead twin is described as either a "fetus compressus†or "fetus papyraceusâ€, depending on the degree of flattening.

With the advent of ultrasound, early loss of a member of a multiple gestation has been identified^4,5 and has been described as "the vanishing twinâ€⁶ or the "vanishing twin syndromeâ€⁷. The vanishing rate has been reported as being as high as 71% of twin gestations diagnosed sonographically before the 10^th week of gestation⁸.When twins were diagnosed between weeks 10 and 15, the loss rate was 63% in this same series. In the 79 patients whose multiple gestation was diagnosed after the 15^th week of gestation, the loss rate was 0%. Robinson and Caines reported a similar loss rate of 53% when twin gestation was diagnosed in the first trimester⁹.

Landy et al¹⁰ advise caution in the interpretation of early ultrasound exams, as artifact can incorrectly imply the presence of an additional gestational sac. Sulak and Dodson¹¹ note the frequent lack of confirmatory histologic evidence in reported vanishing twins. They describe the presence of three gestational sacs after in vitro fertilization in a four week post-conception pregnancy. At the time of genetic amniocentesis at 18.5 weeks, a sonographic exam revealed a singleton pregnancy. Histologic evaluation revealed a single layer of cuboidal amnionic epithelium and a closely apposed layer of chorion in conjunction with a thin bed of chorionic villi underlying the membranes. The sac-like structure was filled with amorphous eosinophilic material. It is their suggestion that the "finding of two layers of amnion-chorion enclosing an area of amorphous nonvillous material, surrounded by viable or degenerative chorionic villi adjacent to the normal amnion-chorion of the normal twin†serves as adequate histologic criteria for a vanished twin.

We present the sonographic appearance of a vanishing twin, diagnosed in the early third trimester. Histologic examination of the placenta was suggestive, although not definitively diagnostic, of the "vanishing twin syndromeâ€. This confirms the need for a diligent histologic examination by an experienced placental pathologist when fetal loss occurs relatively early in gestation.

Case report

A 21-year-old G₂P₁ was referred to the University of New Mexico Medical Center for an ultrasound evaluation after a sonographic exam at an outlying hospital had suggested the possibility of a placental mass. The sonographic exam revealed biometry consistent with the patient"s menstrual dates. No fetal anomalies were noted.

A cystic mass measuring 35 x 33 mm was noted adjacent to the fetal surface of the fundal placenta (fig. 1). Color flow and pulsed Doppler evaluation revealed no evidence of blood flow within the mass. It was felt that the sonographic image was consistent with a "vanishing twin†which would be unlikely to have an adverse outcome on the survivor. Noninvasive surveillance of both the mother and fetus was subsequently initiated.

Figure 1: Sagittal scan showing a fundal placenta. A mass is projecting

At 39 weeks gestation, the patient went into spontaneous labor. She delivered a vigorous 3000g female infant without complication at an outlying hospital. The placenta was later sent to the University of New Mexico. Examination of the placenta revealed an encapsulated 43 x 43 x 25 mm mass on the fetal surface. A 30 x 20 mm mottled white region was also described on the fetal surface. Vessels were noted to be running through the capsule. Microscopic examination of the mass revealed lamellar fibrin deposition (fig. 2) with nests of epithelioid cells and adjacent chorionic villi. Amnion and chorion were not described.

Figure 2: Mass of lamellar fibrin deposition with nests of epithelioid

Discussion

The vanishing twin or "vanishing twin syndrome†has been well described ultrasonographically. This entity is thought to result from the death of one member of a twin or other multiple gestation. The vanishing twin is a relatively common event when multiple gestation is diagnosed early in pregnancy. The prognosis for the surviving fetus is generally excellent, although this will depend upon the etiologic event which resulted in the death of the sibling. There are no known associated anomalies. Given the excellent prognosis, management is conservative.

Differential diagnosis

The sonographic image in our case was highly suggestive of a vanishing twin. The differential diagnosis of the currently described sonographic findings includes chorio­angioma as well as placental cysts. Chorioangioma is the most important entity in the differential diagnosis because of its known relationship to nonimmune hydrops as well as preeclampsia. The lack of blood flow on color Doppler, as well as the histologic evaluation, made the diagnosis of chorioangioma unlikely.

Pathology

Microscopic examination revealed an amorphous mass as well as chorionic villi, similar to that described by Sulak and Dodson. Similarly, the mottled white region on gross inspection was similar to the "yellowish white thickening of the fetal membranes †described by the same authors. However, layers of amnion-chorion were not visualized.

It is difficult to reconcile the ultrasound picture with the gross and microscopic descriptions. The cystic structure on the sonographic image, which likely represents the amnion-chorion, was not evident on examination of the placenta. This was likely to have been a result of in vivo changes or, possibly, suboptimal placental examination. We believe that in the above described case, the absence of microscopic evidence of normal amnion-chorion does not refute the diagnosis of a vanished twin in light of the highly suggestive sonographic image.

References

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2. Benirschke K. Intrauterine death of a twin: Mechanisms, implications for surviving twin, and placental pathology. Semin Diagn Pathol 1993;10:222.

3. Posner AC, Klein, MA. Fetus payraceus. Recognition and significance. Obstet Gynecol 1954;3:106. 4. Levi S, Reimers M. Demonstration echographique de la frequence relativement elevee des grossesses multiples humaines pendant la periode embryonnaire. L"implantation de L"oeuf. Paris, Masson, 1978, pp 295-307.

5. Varma TR: Ultrasound evidence of early pregnancy failure in patients with multiple conceptions. Br J Obstet Gynaecol 1979; 86:290.

6. Jeanty P, Rodesch F, Struyven J: The vanishing twin. J Belge Radiol 1982;63: 526.

7. Landy HJ, Deith L, Deith D. The vanishing twin. Acta Genet Med Gemellol (Roma)1982;31:179.

8. Levi S: Ultrasonic assessment of the high rate of human multiple pregnancy in the first trimester. J C U 1976;4:3.

9. Robinson HP, Caines JS: Sonar evidence of early pregnancy failure in patients with twin conceptions. Br J Obstet Gynaecol 1977;84: 22.

10. Landy HJ, Weiner S, Corson SL,et al: The "vanishing twinâ€: Ultrasonographic assessment of fetal disappearance in the first trimester. Am J Obstet Gynecol 1986;155:14.

11. Sulak LE, Dodson MG: The vanishing twin: pathologic confirmation of an ultrasonographic phenomenon. Obstet Gynecol 1986;68: 811.