[Second trimester sonographic screening for Down's syndrome and other chromosomal abnormalities]

Abstract

Aim: To assess the effectiveness of second trimester sonographic screening for Down syndrome [DS] and other chromosomal abnormalities [ChrA].

Materials and methods: In 1157 singleton pregnancies (15-21 weeks of gestation), including 18 with ChrA (13-DS, 5-other) an expert level detailed sonographic study was performed. The following parameters were analyzed: incidence and type of abnormal sonographic findings in chromosomally normal and abnormal fetuses, sonographic screening sensitivity and specificity (for DS and for other ChrA), positive predictive value [PPV] for DS of the most frequent sonographic markers, negative predictive value [NPV] of the normal genetic sonogram. Data were compared to those for serum screening (results from our previous studies, including all the patients from the present study).

Results: 77.8% of all fetuses with chromosomal abnormalities were identified by ultrasound. The detection rate for DS fetuses (69.2%) was lower and for other aneupliodies (100%)--higher than the detection rate of serum screening. All fetuses with more than one marker were chromosomally abnormal. Increased nuchal fold thickness was highly sensitive for ChrA and also--for DS (66.7% and 60% respectively). The same referred to echogenic bowel (60% and 50% respectively). Pyelectasis was highly sensitive only for DS (23.5%) while shortened femur was not (2.9%). Increased nuchal fold thickness, echogenic bowel and pyelectasis had high individual PPV for fetal DS. It was higher than the PPV of the serum screening positive test result. The NPV of the normal genetic sonogram was lower than the NPV of the screen-negative result from the serum screening.

Conclusions: Second trimester expert fetal sonography is highly specific screening test, but its DS sensitivity is comparatively low. An abnormal genetic sonogram might indicate invasive prenatal testing or may be used to modify DS serum screening risk.