Catalyzes the conversion of progesterone into 20-alpha-dihydroprogesterone (20 alpha-OHP). (17R,20S)-17,20-dihydroxypregn-4-en-3-one + NADP(+) = 17alpha-hydroxyprogesterone + H(+) + NADPH (17R,20S)-17,20-dihydroxypregn-4-en-3-one + NAD(+) = 17alpha-hydroxyprogesterone + H(+) + NADH Monomer. Corpus luteum (large luteal cells). Remains repressed throughout pregnancy and becomes highly and rapidly expressed before parturition. Repressed by prolactin. The N-terminus is blocked. Belongs to the aldo/keto reductase family.