MEASUREMENT OF SERUM CONCENTRATIONS OF INHIBIN-A (ALPHA-BETA(A) DIMER) DURING HUMAN-PREGNANCY

OBJECTIVE The aims were to measure concentrations of inhibin-A (alpha- beta(A) dimer) in peripheral serum during normal human pregnancy, to e stablish which molecular weight form(s) are present in pregnancy serum and to relate the concentrations of inhibin-A to those of oestradiol and progesterone. DESIGN In a retrospective cross-sectional study 211 serum samples collected at 2-week intervals from week 8 to 38 of gesta tion were analysed for inhibin-A by enzyme immunoassay and oestradiol and progesterone by radioimmunoassay. Pooled samples corresponding to first, second and third trimester were subsequently used for fast prot ein liquid chromatography chromatographic analysis of inhibin forms pr esent. PATIENTS Blood samples were obtained from normal pregnant women attending the antenatal clinic. RESULTS Concentrations of inhibin-A i n peripheral serum gradually decreased from 1.76+/-0.15 mu g/l in week 8 of gestation to 0.86+/-0.12 mu g/l in week 16 (P < 0.01). Concentra tions remained low during the second trimester but increased markedly (P < 0.01) during the third trimester reaching a maximal value of 5.68 +/-0.89 mu g/l in week 36. Chromatographic analysis of pooled serum sa mples from the first, second and third trimester showed that the fully processed 31-kDa molecule is the predominant circulating form of inhi bin-A throughout human gestation. Likewise, only the 31-kDa form was i dentified in extracts of term placenta which contained similar to 20 m u g inhibin-A/kg tissue. CONCLUSION Inhibin-A, principally the 31-kDa form, is present in peripheral blood throughout human gestation at con centrations up to 50 times greater than maximum values found during th e spontaneous menstrual cycle (similar to 100 ng/l). The finding of hi ghest serum values during the third trimester and of significant conce ntrations in term placenta firmly support a placental rather than lute al origin for this material.