Modulation of human chorionic gonadotrophin bioactivity during the first trimester of pregnancy

The objective of this study was to evaluate the bioactivity of human chorio nic gonadotrophin (HCG) during first trimester pregnancy. This was done by means of a retrospective analysis of sera from patients with first trimeste r normal intrauterine and ectopic pregnancies, Serum samples were obtained from 38 women with amenorrhoea of <10 weeks, From these, 19 had a normal in trauterine pregnancy (IUP) and 19 an ectopic pregnancy (EP), Cases were all ocated to either low serum immunoreactive HCG (HCGi), intermediate HCGi or high HCGi concentrations (HCGi <5000 mUI/ml, between 5000 and 40 000 mIU/ml and >40 000 mIU/mi respectively). HCGi and oestradiol were measured by enz yme immunoassays and bioactive HCG by the mouse Leydig cell bioassay, All r esults were analysed by analysis of variance and unpaired Student's t-test, There was a significant difference between bioactive to immunoreactive HCG ratios (b/i ratio) between the subgroups of low, intermediate and high HCG i concentrations. Lower b/i ratios were found when HCGi concentrations were high (HCG b/i mean +/- SEM: high subgroup, 0.33 +/- 0.07 versus. low subgr oup: 1.50 +/- 0.12; P < 0.0001), Furthermore, the b/i ratios were inversely correlated with oestradiol (P < 0.0001) and HCGi (P < 0.0001) concentratio ns but not with gestational age, There was no difference in the b/i ratios when comparing IUP with EP. It is concluded that, in first trimester pregna ncies, there is a likely modulation of HCG bioactivity which is inversely c orrelated with HCGi and oestradiol concentration. The underlying mechanisms and their physiological relevance remain to be elucidated.