Cribado de aneuploidía en gestación gemelar: resultados de la aplicación del test combinado

Objective

To evaluate the effectiveness of the Combined Test for trisomy 21 screening in twin pregnancies. To assess the performance of biochemical markers and nuchal translucency (NT) measurement in pregnancies with euploid fetuses and in twin pregnancies with one or two affected fetuses. To compare the value of markers according to chorionicity and the mode of conception.

Material and methods

Retrospective study including 161 twin pregnancies. Maternal serum fß-hCG and PAPP-A were determined at 8 to 12 weeks and fetal NT was measured at 11 to 14 weeks. The individual risk of trisomy 21 was calculated in each fetus using the Combined Test. In monochorionic pregnancies, the single risk for the pregnancy was obtained with the largest NT. An invasive diagnostic procedure was offered when the risk was 1:250 or more in one or both of the fetuses.

Results

All trisomy 21 pregnancies were identified (three pregnancies and four fetuses) by the combined testfor a false-positive rate of 6.4% of pregnancies and 3.5% of fetuses. The median fß-hCG level, expressed in MoM, was 1.72 and the median PAPP-A level was 2.01. The median NT was 1.05 MoM. Both fß-hCG and PAPP-A levels were significantly decreased in monochorionic pregnancies and PAPP-A was significantly decreased in pregnancies resulting from assisted reproduction. No significant differences were observed in NT measurement between monochorionic and dichorionic fetuses or between those conceived naturally or by assisted reproduction.

Conclusions

The combined test shows high sensitivity and specificity in screening for trisomy 21 in twin pregnancies. The differences obtained in the biochemical markers according to chorionicity or the mode of conception require confirmation in further studies with a larger number or cases.