The significance of a positive second trimester serum screen for trisomy 18

We designed this study to estimate the proportion of fetuses in pregnancies with positive second trimester serum screens for trisomy 18 who actually have trisomy 18, to estimate the proportion of women with trisomy 18 who have a negative serum screen, and to assess the role of ultrasound in the diagnosis of trisomy 18.

Retrospective study of two cohorts of pregnant women in 2004 and 2005: (1) those with a second trimester serum screen positive for trisomy 18 and (2) those with fetuses having trisomy 18.

There were 93 women with positive serum screens for trisomy 18. Of these, only three had a fetus with trisomy 18. There were five other cases of trisomy 18, three of which had a negative second trimester serum screen for trisomy 18. All fetuses with trisomy 18 had multiple major structural abnormalities detected on targeted genetic sonography.

A positive second trimester serum screen has a poor sensitivity and poor prediction for trisomy 18. Trisomy 18 is highly unlikely if a woman with a positive screen for trisomy 18 has no fetal abnormalities detected on targeted genetic sonography. Women with a positive second trimester serum screen for trisomy 18 should be offered genetic sonography, and the practice of routine amniocentesis for all women with a positive screen should be discouraged when targeted genetic sonography is available.