Does increased nuchal translucency indicate a fetal abnormality? A retrospective study to clarify the clinical significance of nuchal translucency in Japan

The results of a chromosomal test by genetic amniocentesis in 58 cases with anincreased nuchal translucency (NT; ?3 mm thickness) revealed 47 cases showing anormal karyotype (81%) and 11 cases (19%) showing an abnormal karyotype. However,the cases of a normal karyotype with increased NT also included those with fetalabnormalities. Among the 49 cases in which NT was observed during the 1st trimesterand then subsequently disappeared, chromosomal abnormalities were observed in 5cases, and fetal abnormalities other than chromosomal abnormalities were observed in 2cases. Meanwhile, all 9 cases in which an increased NT remained or in which NTcontinued to increase in size during the 2nd trimester were diagnosed to have cystichygroma, and chromosomal abnormalities were found in 6 cases (67%). It should benoted that the shape of increased NT includes NT with a notch (Notched NT) and NTwithout a notch (Smoothed NT). Among the 20 cases of Notched NT, chromosomalabnormalities were observed in 8 cases (40%), and cystic hygroma was observed in 9cases (45%). On the other hand, among the 38 cases of Smoothed NT, chromosomal abnormalities were observed in 3 cases (7.9%), but no cystic hygroma was observed.Our results clarified that increased NT does not always indicate a fetalabnormality. Whether the thickness of NT should be measured as screening of fetalabnormalities remains controversial. However, an increased NT may be detected bychance, because a maternal-fetal medical examination using ultrasonography is usuallyperformed in Japan. It is therefore considered to be extremely important to establish asystem in which cases are referred to obstetricians who are licensed clinical geneticspecialists in order to obtain appropriate genetic counseling whenever an increased NTis clinically observed