Background: When confronting a dizygotic pregnancy with one fetus affected with chromosomal aberrations, most couples would opt for selective termination of the affected twin.
Case: Routine genetic amniocentesis was performed for an in vitro fertilization-embryo transfer twin pregnancy at 18 weeks' gestation due to advanced maternal age. After two weeks, cytogenetic analysis revealed that both twins were male and one was affected with trisomy 21. At that time, ultrasound examination could not tell them apart with certainty. With the aid of interphase fluorescence in situ hybridization (FISH), we had no trouble locating the affected twin and performed feticide successfully with an intracardiac potassium chloride injection. At 37 weeks of gestation, a normal male was delivered along with a macerated trisomy 21 fetus.
Conclusion: In a dizygotic twin pregnancy discordant for chromosome aberrations, when ultrasound cannot distinguish the affected twin, performing interphase FISH with an appropriate chromosome probe proves very useful in quickly and accurately locating the chromosomally abnormal twin for selective termination.