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The peripheral plasma levels of 20alpha-dihydroprogesterone (20alpha-DHP), progesterone (P) and 17-hydroxyprogesterone (17-OHP) were measured by radioimmunoassay techniques in 440 samples during normal human pregnancy between weeks 4 and 41. The levels of 20alpha--DHP in plasma from the 4th to the 6th week were between 6.0 and 6.6 ng/ml. From then until the 21st week the average plasma 20alpha-DHP concentrations remained at the same level between 4.0 and 6.3 ng/ml; they then rose significantly to and beyond term, levels reaching over 40 ng/ml. The range of mean plasma concentration of P during the first trimester of pregnancy fell to a nadir in the 9th week (170 ng/ml) then rose with increased gestation until the 39th week (190.4 ng/ml) followed by a slight and not significant drop. Single measurements of plasma 17--OHP from the 4th to the 6th week of pregnancy gave value between 2.8 and 3.6 ng/ml, but from the 7th week the mean plasma 17--OHP levels gradually declined, then from week 30 the 17-OHP concentration increased to reach a mean level of 7.63 ng/ml in the 41st week. The ratio P/20alpha--DHP increased from the 4th (3.5:1) to the 24th week (15.6:1) and then decreased from 25th week (7.9:1) towards term (3.2:1).