Correlation of Second-Trimester Sonography and Biochemical Markers

Combining biochemical and sonographic data is an efficient means of screening for Down syndrome in the first trimester, but it is not clear whether this holds for the second trimester as well. This study evaluated 7 sonographic markers and the triple test of maternal serum biochemical markers in 2183 women with seemingly normal singleton fetuses who had sonography at 14 to 22 weeks gestation. The triple test included estimates of human chorionic gonadotropin (hCG), alpha-fetoprotein, and estriol. Sonography assessed nuchal fold thickness, humerus length, femur length, renal pyelectasis, hyperechoic bowel, echogenic intracardiac focus, and choroid plexus cysts. Maternal serum levels of hCG correlated significantly (although weakly) and negatively with both femur length and humerus length. There were no other correlations that were significant at the 1% level between femur or humerus length and the biochemical markers. Nuchal fold thickness did not correlate significantly with any biochemical marker. At the 5% level, hCG levels were lower when an echogenic intracardiac focus was found. Hyperechoic bowel tended to correlate with higher hCG and alpha-fetoprotein levels. Mean maternal age did not correlate significantly with log-transformed multiples of the mean (MOM) for hCG, alpha-fetoprotein, estriol, nuchal fold thickness, or humerus length. Maternal age did correlate negatively with the log-transformed MOM for femur length. In this control population, sonographic and biochemical markers of Down syndrome, evaluated in the second trimester, were largely independent of one another. It follows that these markers may be used as independent modifiers when assessing the risk of fetal Down syndrome.