DOWN-SYNDROME AND NEURAL-TUBE DEFECT SCREENING - THE VALUE OF USING GESTATIONAL-AGE BY ULTRASONOGRAPHY

OBJECTIVE: Our goal was to determine whether gestational age should be based on ultrasonographic evaluation or last menstrual period data in the interpretation of second-trimester maternal serum screening for D own syndrome and open neural tube defects. STUDY DESIGN: Initial and r evised screen-positive rates and detection rates were reviewed for wom en undergoing triple-marker testing (maternal serum a-fetoprotein, hum an chorionic gonadotropin, and unconjugated estriol). The study popula tion consisted of >24,000 women at 15.0 to 21.9 weeks' gestation with approximately 60% of test interpretations based on ultrasonographic ev aluation of gestational age. Gestational age and screening results wer e compared for 24 Down syndrome pregnancies in which both ultrasonogra phy and last menstrual period dating were available. RESULTS: Both ini tial and revised screen-positive rates for Down syndrome were signific antly lower when ultrasonographic data were used compared with last me nstrual period dating. The detection rate for Down syndrome appeared t o be higher with ultrasonographic dating (approximately 76% vs 60% for last menstrual period dating). Down syndrome fetuses had a significan tly shorter gestational age when evaluated by ultrasonography (relativ e to last menstrual period dating], but a similar trend was also seen in control pregnancies. Initial and revised screen-positive rates for open neural tube defects were higher for women who had received an ult rasonographic examination compared with the rates for those women refe rred with only last menstrual period data. The detection rates for ope n neural tube defects were similar for both methods of pregnancy datin g. CONCLUSION: By use of ultrasonographic measurement of gestational a ge, the number of amniocenteses performed to detect Down syndrome can be substantially reduced while detection rates are maintained or impro ved.