'Dual positivity' for neural tube defects and down syndrome at maternal serum screening: gestational outcome

To evaluate the gestational outcome of pregnancies screen-positive for both neural tube defects (NTD) and Down syndrome (DS) ('dual positivity').

Among 10,667 mid-trimester women screened for DS and NTD with alpha-fetoprotein (AFP), unconjugated estriol (uE3), and human chorionic gonadotropin (hCG), delivered up to July 1996, we have selected cases with both an unexplained AFP value > or = 2.5 multiples of median (MoM) and a DS risk > or = 1:250. All these pregnant women were managed with amniocentesis and/or CVS, ultrasound scans, and Doppler velocimetry. We have collected all data about the gestations with 'dual positivity' and no obvious explanation for these findings (cases with fetal malformations related to raised AFP).

Twelve women (1.1:1,000) showed unexplained 'dual positivity'. Abnormal karyotypes were found in 3 fetuses, and pregnancies were terminated: there were 2 triploidies with partial hydatiform mola, and 1 DS. In 9 cases the fetal karyotype was normal, but a confined placental trisomy 16 was found in 4. Of the 9 continuing gestations, 8 displayed fetal growth retardation (FGR). One gestation ended with fetal death at 27 weeks. All 9 fetuses were morphologically normal, and 8 were small for gestational age.

'Dual positivity' at NTD/DS screening may anticipate pregnancy complications. The finding of trisomy 16 confined to the placenta and FGR in 4 cases suggests that at least some fetuses with growth restriction may suffer from a distinct placental disease. Maternal serum screening may have implications different from DS and NTD, as demonstrated by the 2 cases with triploidy and incomplete hydatiform mola, the 4 cases with placental trisomy 16, and the 4 cases of FGR of the 5 fetuses without chromosome abnormalities. As the pathologic outcome of these pregnancies is more important than the mere serum screening results, we feel that these cases need a strict work-up, including CVS, amniocentesis and ultrasound studies to better address the obstetrical management.