LC-MS/MS quantitation of the primary reduced metabolites of progesterone in serum during the third trimester of human pregnancy reveals a potential role for 20β-hydroxyprogesterone and 5β-dihydroprogesterone in functional progesterone withdrawal.

Progesterone (P4) is an essential steroid hormone synthesised by the placenta required for the maintenance of pregnancy. In humans, the metabolism of P4 has been implicated in functional P4 withdrawal prior to parturition. We have developed a novel liquid chromatography tandem mass spectrometry (LC-MS/MS) method for quantitation in human serum of pregnenolone, P4 and its four primary reduced metabolites; 20α-hydroxyprogesterone (20α-OHP), 20β-hydroxyprogesterone (20β-OHP), 5α-dihydroprogesterone (5α-DHP) and 5β-dihydroprogesterone (5β-DHP). Following solid phase extraction, chromatographic baseline separation of each steroid was achieved using a biphenyl stationary phase within a 10.0 min runtime, followed by MS detection on a Sciex 6500+. The LC-MS/MS method was validated in accordance with published guidelines, confirming acceptable analytical performance pertaining to linearity, imprecision, accuracy, sensitivity, matrix effects, specificity and carryover. The method was applied to a large cohort of third trimester pregnancies with verified uncomplicated neonatal outcomes. Maternal circulating concentrations of P4, 20α-OHP, 20β-OHP, and 5α-DHP positively correlated with fetal gestational age. The ratio of P4:20β-OHP declined significantly throughout the third trimester, whilst the ratio of P4:5β-DHP increased at full term from 40 weeks' gestation. These findings may indicate a substantive role for β-reduction of P4 in the mechanics of functional P4 withdrawal, either via depletion of the overall pool of bioactive P4 or competitive binding of these metabolites to the P4 receptor in maternal and fetal tissue. Additionally, detailed characterisation of the normal maternal steroidome will facilitate the study of dysregulated placental steroidogenesis, which has been implicated in the pathogenesis of the major obstetric syndromes causing poor pregnancy outcomes.