First‐trimester biochemical screening for fetal chromosome abnormalities and neural tube defects

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Abstract
Alpha‐fetoprotein (AFP), unconjugated oestriol (UE3), intact human chorionic gonado‐trophin (intHCG), and the free β subunit of chorionic gonadotrophin (FβHCG) were investigated in a series of 21 chromosomally abnormal and 14 open neural tube defect pregnancies ascertained from a series of 14 000 prospectively collected maternal serum samples at 6–14 weeks' gestation. In 16 cases of Down's syndrome, significant reductions were found for AFP (0.65 multiples of the normal median) and UE3 (0.67 MOM). IntHCG levels were unaltered (0.97 MOM) but a significant increase was found for FβHCG (1.96 MOM). Significant correlations were found for AFP and UE3 in the controls and for int HCG and FβHCG in both the control and the Down's syndrome pregnancies. In a group of five trisomy 18 pregnancies, median MOMs were for AFP 0.71, for UE3 0.34, for intHCG 0.27, and for FβHCG 0.15. None of 13 pregnancies with open neural tube defects at 8‐13 weeks gestation had elevated maternal serum AFP levels, whereas matched second‐trimester samples from the same pregnancies at 16‐18 weeks gestation all had significantly elevated AFP levels. Thus, biochemical screening for chromosome abnormalities may be practicable in the first trimester using free β human chorionic gonadotrophin in combination with AFP and maternal age. However, a separate screening protocol using AFP at 15‐18 weeks gestation would still be required for effective detection of neural tube defects.