Human chorionic gonadotrophin and subunit composition of maternal serum and coelomic and amniotic fluids in the first trimester of pregnancy

ABSTRACT Sixteen matched samples of first trimester amniotic fluid (AF), extraembryonic coelomic fluid (EECF) and maternal serum (MS) were assayed for intact human chorionic gonadotrophin (hCG) and free subunits. Total β-hCG (free β-subunit and intact hCG) levels in the EECF (median 410 kIU/l) were 61 times greater than levels in AF (median 6·73 kIU/l) and 2·8 times greater than in MS (median 141·5 kIU/l). Levels of intact hCG in the EECF (median 245 kIU/l) were 142 times greater than in AF (median 1·73 kIU/l) and 1·6 times greater than in MS (median 157 kIU/l). Free α-subunit levels in EECF (median 17·3 mg/l) were 66 times greater than in AF (median 0·262 mg/l) and 12 times greater than in MS (median 1·3 mg/l). Virtually all of the total β-hCG immunoreactivity in MS can be attributed to intact hCG, but only 60% of total β-hCG in the EECF and 20% of that in AF can be accounted for by the intact hormone. In both EECF and AF the free α-subunit was a major constituent; on a molar basis the ratio of free α:free β:intact hCG was 1:1·2:0·3 in AF, 1:0·6:0·5 in EECF and 1:0:5 in MS. Chromatography of MS, EECF and AF on Sephadex G-100 confirmed the hCG and subunit composition of the fluids. On the basis of these findings it seems likely that previous studies showing very high levels of hCG in AF during the first trimester may have incorrectly sampled the EECF. In reality, the levels of total hCG (and free subunits) are low in the AF, and only 20% is intact hCG. In both AF and EECF the free subunits may have been derived by dissociation of intact hormone, or possibly by independent synthesis. These and other findings suggest that either the amnion acts as a barrier to the transfer of proteins or that there may be dynamic removal from this compartment. By contrast, the EECF might act as a relatively stable reservoir for these proteins. Journal of Endocrinology (1992) 135, 563–569