Screening for trisomy 21 in monochorionic twins by measurement of fetal nuchal translucency thickness.

OBJECTIVE To determine whether in screening for trisomy 21 by measurement of fetal nuchal translucency (NT) thickness in monochorionic twin pregnancies it is preferable to use the higher, smaller or average NT. METHODS We retrospectively examined 769 monochorionic twin pregnancies that had undergone NT screening. The selection criteria were that first, in each pregnancy both fetuses were alive at the 11 to 13 + 6-week scan and second, the fetal karyotype had been determined by prenatal invasive testing or the pregnancy outcome was known. In each pregnancy the risk for trisomy 21 was calculated by a combination of maternal age and fetal NT for crown-rump length (CRL). Three estimates of risk for each pregnancy were made using the higher, smaller and average NT and these were compared for detection and false positive rates. RESULTS The median maternal age was 33 (range, 16-45) years, the CRL was 62 (range, 45-84) mm and gestational age was 12 (range, 11 to 13 + 6) weeks. Either the fetal karyotype was normal, or phenotypically normal babies were born, in 761 cases. The karyotype was abnormal in eight cases, including six with trisomy 21. The estimated risk using the higher, smaller and average NT was 1 in 300 or more in 6 (100%), 4 (66.7%) and 6 (100%) of the trisomy 21 pregnancies and in 148 (19.4%), 57 (7.5%) and 106 (13.9%) of the normal pregnancies. For a detection rate of 100%, the false positive rates using the higher, smaller and average NT would be 5.1%, 45.9% and 4.2%, respectively. CONCLUSION In monochorionic twins, effective screening for trisomy 21 is best provided by using the average NT measured in the two fetuses.