A re-assessment of biochemical marker distributions in T21 affected and unaffected twin pregnancies in the first trimester

OBJECTIVE: To estimate the difference between levels of the two biochemical markers pregnancy-associated plasma protein-A (PAPP-A) and maternal serum free β-human chorionic gonadotropin (free β-hCG) in twin pregnancies relative to singleton pregnancies and establish improved screening procedure for chromosomal anomalies such as trisomy 21 in twin pregnancies. METHODS: 4843 unaffected and 47 trisomy 21 affected twin pregnancies were included in the study. Chorionicity specific medians were generated for PAPP-A and free β-hCG from gestational age 8 to 14 weeks. Multiple of the median (MoM) values for each of the biochemical markers were calculated. Detection rates and false-positive rates were estimated for screening tests incorporating Nuchal Translucency (NT) and maternal age, with and without biochemistry. RESULTS: Medians for the two biochemical markers for monochorionic and dichorionic twins in unaffected pregnancies show a gestational age specific increase relative to the singleton medians. Allowing for gestation and chorionicity, twin pregnancies affected with trisomy 21 had higher levels of free β-hCG and lower levels of PAPP-A. Adding biochemistry into the risk assessment increased the detection rate for fetal trisomy 21 in dizygotic twin pregnancies from 78% to 90%, and decreased the false positive rate from 8.0% to 5.9%. CONCLUSION: Generation of chorionicity specific medians for the biochemical markers and their use in risk assessment can improve the performance of first trimester screening for chromosomal abnormalities in twins to a level comparable to that in singleton pregnancies. Copyright © 2010 ISUOG. Published by John Wiley & Sons, Ltd.