Discordance of fetal genotype or phenotype in monozygotic twins: A report of 2 cases

BACKGROUND: Discordance of fetal genotype or phenotype in a monozygotic twin pregnancy is rare. CASE: In case 1, a 28-year-old woman at 15 weeks' gestation was found to have a dichorionic twin pregnancy with 1 fetus affected with hydrop fetalis. The result of chromosomal study showed that the structurally normal fetus was 46,XY and that the hydropic fetus was 45,X. One week after selective termination of the hydropic fetus at 19 weeks' gestation, the cotwin died in utero. In case 2, a 30-year-old woman at 20 weeks' gestation, was found to have a monochorionic twin pregnancy with 1 fetus presenting with omphalocele. The result of chromosomal study showed that both fetuses were 46,XX. The fetus with omphalocele died in utero at 29 weeks' gestation, and the normal cotwin was delivered later due to fetal distress. Analysis by short tandem repeat markers in both cases indicated that they were monozygotic twins. CONCLUSION: These cases emphasize the importance of zygosity/chorionicity identification in twin pregnancy even though discordance of fetal genotype or phenotype was found. In monozygotic monochorionic twins, the normal cotwin is at risk for an adverse fetal outcome after 1 spontaneous intrauterine fetal death or selective termination. In monozygotic dichorionic twins, the risk of intrauterine fetal demise of the cotwin after selective termination still exists.