BJOG : an international journal of obstetrics and gynaecology
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To determine how first trimester detection rates for Down's Syndrome vary with maternal age and to calculate the predictive value of an increased risk report at various maternal ages. ⋯ As with second trimester biochemical screening, the detection rate and false positive rate vary considerably with age. However, detection rates across all ages are significantly higher than with second trimester screening. The risk of a positive screening result being a Down's pregnancy is considerably greater than with second trimester screening with an average probability of 1:29, compared with 1:55 in the second trimester. This information may be useful in counselling women with an increased risk result in first trimester screening.