Utilization of second-trimester genetic sonogram for detection of trisomy 21 and its role in influencing women's decision to undergo amniocentesis

OBJECTIVE: To determine the utilization rate of second-trimester genetic soiiogram in patients at increase risk for fetal trisomy 21 and its role in influencing the decision for amniocentcsis in this group of patients. STUDY DESIGN: From 11/1/93 to 7/31/96 a second trimester genetic sonogram or straight genetic anmioceiitesis have been offered to all women at increased risk (at least 1:274) for trisomy 21 due to advanced materna! age (AMA), (at least 35 years) or abnormal serum biochemistry. During the genetic sonogram the following ultrasound (U/S) markers for aneuploidy were evaluated: structural anomaly, short femur, short humérus, pyelectasis, nuchal fold thickening, echogenie bowel, choroid plexus cyst, hypoplastic middle phalanx of the fifth digit, wide space between the first and second toes, and 2-vesscl umbilical cord. In our experience genetic sonogram has had a sensitivity of 86% and a false positive rate of 14% in detecting fetal trisomy 21. If the results of the genetic sonogram were normal (absence of all U/S markers) patients were counseled that their risk for trisomy 21 was reduced by at least 50% during the first 20 months of the study, or 75% during the last 13 months of the study. Patients who desired genetic amniocetitesis after the results of the genetic sonogram had the procedure done immediately. The utilization rates of genetic sonogram for each of the years 1993-96 were determined. Statistical analysis included descriptive statistics and logistic regression analysis which used the decision for amniocentesis as the dependent variable and the following factors as independent variables: maternal age, gravitity, parity, individual risk prior to amniocentesis, risk factor, gestational age, multiple gestation, inability to visualize all markers, number of abnormal L'/S markers and individual performing the exam. Results arc expressed in odds ratios (OR) and 95% confidence intervals (CI) RESULTS: During the study period 1,851 patients were referred to our unit for genetic prenatal diagnosis due to AMA ( 1,320 or 71%), abnormal serum biochemistry (438 or 24%) or other (nonimagiiig) indications (93 or 5%), Of the 1,851 patients, 1,319 (71%) chose straight genetic amniocentesis and 532 (29%) chose a genetic sonogram as a first option. The utilization rates of genetic sonogram through the years 1993-96 were as follows: year 1993 (0/477 or 0%); 1994 (84/495 or 17%); 1995 (251/523 or 48%) and Jan-Jul 1996 (197/356 or 55%). The amniocentesis rate among the 532 patients who chose genetic sonogram was as follows: normal U/S (no markers) 17/481 (3.5%), 1 U/S marker present 11/39 (28%) and at least 2 U/S markers present 9/12 (75%). After adjusting for year and individual performing the U/S exam, logistic regression analysis showed that the most important factors associated with the women's decision to undergo amniocentesis were: >2 U/S markers present (OR = 96.3; 95% CI = 23.7-513.1), onlv 1 U/S marker present (OR = II.8; 95%, CI = 4.7-29.1) and abnormal serum biochemistry (OR = 4,2; 95% CI = 1.4-13.6). CONCLUSION: Offering genetic sonogram to patients at increased risk for fetal trisomv 21 is associated with a high utilization rate. The increasing utilization trend (55% during the most recent 7 months) in conjunction with the fact lhat an abnormal sonogram was the most influential factor in women's decision to undergo genetic amniocentesis, suggest that genetic sonogram services for detection of trisomy 21 should be added to the armamentarium of all prenatal diagnostic centers.