FETAL KARYOTYPING FOR CHROMOSOME-ABNORMALITIES AFTER AN UNEXPLAINED ELEVATED MATERNAL SERUM ALPHA-FETOPROTEIN SCREENING

Objective: To study the chromosome abnormality rate among women with e levated levels of maternal serum alpha-fetoprotein (MSAFP) and the typ es of chromosome abnormalities in this population, and to compare this rate with reports in the literature and the rate observed in the gene ral population. Methods: We studied 8097 women who chose to undergo am niocentesis and fetal karyotyping after having an elevated MSAFP test of 2.5 multiples of the median (MOM) or higher. All abnormal karyotype s were reviewed and grouped according to whether the elevated MSAFP va lue could be explained by a ventral wall or neural tube defect, Result s: The overall chromosome abnormality rate was 13.83 per 1000 amniocen teses. The rate in the ''unexplained'' group was 10.92 per 1000 amnioc enteses. Just over half (53%) of the abnormal karyotypes were autosoma l anomalies, and 47% were sex chromosome abnormalities. The autosomal aneuploidies observed most frequently were triploidy and trisomy 13. T he sex chromosome abnormalities observed most frequently were the XXY and XYY karyotypes. Conclusion: Women who have unexplained elevated MS AFP values of 2.5 MOM or greater have a twofold increase in the rate o f chromosome abnormalities in their fetuses compared with the general population (P less than or equal to .001). This rate is consistent wit h other studies that used a 2.5 MOM cutoff. Studies that used a 2.0 MO M cutoff have reported chromosome abnormality rates that do not vary f rom general population estimates.