The role of ultrasonographic markers for trisomy 21 in women with positive serum biochemistry.

Objective: To assess the value of particular markers detected by second trimester ultrasound examination among those women whose fetuses were shown to be at increased risk of Down's syndrome on the basis of biochemical screening.

Design: A retrospective study of 459 pregnancies.

Setting: Fetal Medicine Unit, Royal Free Hospital.

Participants: Four hundred and fifty-nine pregnant women, including four twin pregnancies, registered at the Royal Free Hospital, who were considered screen positive (risk > 1:250) based on the results of mid-trimester biochemical markers (maternal serum free beta human chorionic gonadotrophin and alpha-fetoprotein).

Main Outcome Measures: The ultrasound markers that were examined included structural defects, shortened femur length, echogenic bowel, dilation of the renal pelvis and choroid plexus cysts. The likelihood ratios for trisomy 21 for each of these markers were calculated.

Results: Of the 463 fetuses which were screen positive, 449 (97%) had a normal karyotype detected by amniocentesis (n = 344) or postnatal follow up (n = 105). Fourteen fetuses had an abnormal karyotype including 11 (2.4%) with trisomy 21. Ultrasound markers were found in 9/11 (81.8%) fetuses with trisomy 21, compared with 44/449 (9.8%) with a normal karyotype. Detection of one or more ultrasonographic markers in a screen positive pregnancy increased the risk of trisomy 21 by a likelihood ratio of 8.4, and the absence of such markers decreased the risk by a likelihood ratio of 0.2. The risk was considerably increased when the presence of two or markers were detected (likelihood ratio 41). In trisomy 21 fetuses the two most commonly detected markers, shortened femur and dilation of the renal pelvis, had likelihood ratios of 49.3 and 20.5, respectively. Choroid plexus cysts were detected in 27 of the normal karyotypic fetuses compared with none of those with trisomy 21.

Conclusion: The presence or absence of abnormal ultrasonographic markers can significantly change the risk of Down's syndrome among pregnant women already found to have abnormal serum biochemistry. This data may be useful in counselling such women.