Early transvaginal measurement of cephalic index for the detection of down syndrome fetuses.

OBJECTIVE

The aim of this study was to produce normal cephalic index values by transvaginal scan early in pregnancy and to evaluate the screening utility of this measurement for the identification of fetuses at risk for Down syndrome.

METHOD

In this prospective cross-sectional study, transvaginal high-resolution sonography was performed between 9 and 16 weeks of gestation in 1,087 euploid fetuses and 36 Down syndrome fetuses. Measurements of the cephalic index, calculated as the ratio biparietal diameter/occipito-frontal diameter, were plotted against gestational age.

RESULTS

The cephalic index was found to show fairly constant values throughout the period evaluated. The best correlation with gestational age was described by a linear correlation. All the measurements obtained in Down syndrome fetuses were within the normal ranges.

CONCLUSION

Our data show that in early pregnancy the cephalic index cannot be considered a useful tool in the detection of fetuses at risk for Down syndrome.