Ultrasonographic markers for chromosomal abnormalities in women with negative nuchal translucency and second trimester maternal serum biochemistry.

OBJECTIVE: To analyze the value of second trimester ultrasound examination among those women whose fetuses were indicated to be at low risk of chromosomal anomalies on the basis of both first trimester nuchal translucency measurement and second trimester biochemical screening. METHODS: A retrospective study of 5500 pregnancies carried out at the fetal medicine unit, Royal Free Hospital. During a period of over 3 years 5500 pregnancies underwent a first trimester scan and nuchal translucency measurement which enabled the detection of 62% (20 of 32) of all chromosomal anomalies. From the remaining pregnancies that underwent second trimester biochemical screening, 3548 were considered negative (risk < 1:250; using maternal serum free beta human chorionic gonadotrophin and alpha fetoprotein). The ultrasound markers that were examined were: shortened femur length, echogenic bowel, pyelectasis, choroid plexus cysts and echogenic intracardiac foci. The likelihood ratios for chromosomal aneuploides for each of these markers were calculated. RESULTS: Of the 3548 screen negative pregnancies, 3541 (99.8%) had a normal karyotype. Seven (0.2%) fetuses had an abnormal karyotype including four (0.11%) with trisomy 21, one with trisomy 18 and two with 47XXY. Second trimester ultrasound markers were found in two of the five (40%) with severe chromosomal anomalies compared to 184 of 3541 (5.2%) with normal karyotypes. Detection of one or more ultrasound markers in a screen negative pregnancy increased the possibility of chromosomal aneuploidy and a negative ultrasound decreased the risk by a likelihood ratio of 0.6 (95% confidence interval, 0.3-1.3). The risk was considerably increased when two or more markers were detected and we would recommend karyotyping under these circumstances. CONCLUSION: This preliminary data indicates a possible role for abnormal ultrasound markers in assessing the risk of chromosomal abnormalities in patients considered to be at low risk by nuchal translucency and serum screening. However analysis of a much larger study group will have to be conducted to assess the significance of individual markers.