Total amount of circulating human chorionic gonadotrophin alpha and beta subunits in first trimester trisomies 21 and 18.

The aim of this study was to evaluate the variations in the balance between total (free and combined) circulating alpha and beta subunits of human chorionic gonadotrophin (hCG) in trisomy 21 and 18. Maternal serum samples were collected at 10 and 11 weeks of gestation from 22 singleton pregnancies with trisomy 21 (n = 17) and trisomy 18 (n = 5) and 66 chromosomally normal controls, matched for gestational age. The hCG and free alpha and beta subunits circulating levels were measured using specific immunoradiometric assays and converted in a common unit system obtained using calibration of the assays with intact and thermally dissociated purified hCG preparation. In trisomy 21, the only significant difference from controls was in the free beta hCG level which was increased. In trisomy 18, intact hCG, free beta hCG as well as total alpha hCG and total beta hCG levels were significantly lower whereas the free alpha hCG level was significantly higher than in controls. The decrease in total beta hCG was more pronounced than the decrease in total alpha hCG resulting in a significant increase in the total alpha- to beta hCG subunit ratio in trisomy 18. These findings suggest some modifications in the biosynthesis and/or release rates of the hCG subunits in these trisomies.