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 bioc hemical screening. Design A retrospective study of 459 pregnancies. Se tting Fetal Medicine Unit, Royal Free Hospital. Participants Four hund red and fifty-nine pregnant women, including four twin pregnancies, re gistered at the Royal Free Hospital, who were considered screen positi ve (risk > 1:250) based on the results of mid-trimester biochemical ma rkers (maternal serum free beta human chorionic gonadotrophin and alph a-fetoprotein). Main outcome measures The ultrasound markers that were examined included structural defects, shortened femur length, echogen ic bowel, dilation of the renal pelvis and choroid plexus cysts. The l ikelihood ratios for trisomy 21 for each of these markers were calcula ted. Results Of the 463 fetuses which were screen positive, 449 (97%) had a normal karyotype detected by amniocentesis (n = 344) or postnata l follow up (n = 105). Fourteen fetuses had an abnormal karyotype incl uding 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 n ormal karyotype. Detection of one or more ultrasonographic markers in a screen positive pregnancy increased the risk of trisomy 21 by a like lihood ratio of 8.4, and the absence of such markers decreased the ris k by a likelihood ratio of 0.2. The risk was considerably increased wh en the presence of two or markers were detected (likelihood ratio 41). In trisomy 21 fetuses the two most commonly detected markers, shorten ed 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 trisom y 21. Conclusion The presence or absence of abnormal ultrasonographic markers can significantly change the risk of Down's syndrome among pre gnant women already found to have abnormal serum biochemistry. This da ta may be useful in counselling such women.