Background As maternal age advances, the risk of fetal Down'ssyndrome increases. Pregnant women 35 years of age or olderare routinely offered amniocentesis because of this risk. Recently,maternal serum markers have been reported to be useful in screeningfor Down's syndrome, primarily in younger women. We thereforeinvestigated whether offering amniocentesis only to selectedwomen 35 years of age or older who were identified by screeningmeasurements in serum might prove a useful alternative to thecurrent practice.
Methods We studied 5385 women with singleton pregnancies whowere 35 years of age or older and were undergoing routine amniocentesis.Along with information about the pregnancy, we obtained a serumsample for measurement of alpha-fetoprotein, unconjugated estriol,and human chorionic gonadotropin. Individual estimates of therisk of Down's syndrome in the fetus were calculated for eachpregnancy before the karyotype was known.
Results If amniocentesis had been reserved for the women calculatedto have a risk greater than 1 in 200 of having a fetus withDown's syndrome, 48 of the 54 cases of Down's syndrome (89 percent)would have been identified; 25 percent of the unaffected pregnancieswould also have been identified as being at high risk for Down'ssyndrome (false positives). Seven of 15 fetuses (47 percent)with other trisomies, 11 of 25 (44 percent) with sex aneuploidy,and 1 of 9 (11 percent) with miscellaneous chromosomal abnormalitieswould also have been detected. In practice, such screening wouldhave made 75 percent of the amniocenteses unnecessary, alongwith a proportion of the amniocentesis-associated fetal losses.If the cutoff for the risk of Down's syndrome were set higherthan 1 in 200, both the rate of detection and the false positiverate would be lower. Conversely, these rates would be higherif the cutoff were set lower.
Conclusions Prenatal screening of serum to generate individualestimates of the risk of Down's syndrome in the fetus can providea basis for decision making in the cases of women 35 years ofage or older, as it does in younger pregnant women, and is analternative to current testing practices. .
Source Information
From the Foundation for Blood Research, Scarborough, Me. (J.E.H., G.E.P., G.J.K.); the California Department of Health Services, Genetic Disease Branch, Berkeley (G.C.C., L.S.L.); and the California Public Health Foundation, Berkeley (P.A.B.).
Address reprint requests to Dr. Haddow at the Foundation for Blood Research, Scarborough, ME 04074.
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