Screening for trisomy 21 by maternal age, fetal nuchal translucency thickness, free beta‐human chorionic gonadotropin and pregnancy‐associated plasma protein‐A

A beneficial side effect of first-trimester combined screening for trisomy 21 is the detection of a high proportion of fetuses with trisomy 18. If an algorithm for trisomy 18 in addition to the one for trisomy 21 is used, more than 95% of trisomy 18 fetuses can be detected with a minor increase of 0.1% in the overall FPR.