A monthly cycle during third trimester pregnancy in the serum concentrations of progesterone and aldosterone and the urinary excretion rate of potassium.

Eleven healthy women were examined three times a week during the third trimester. A mathematical model (parabolas overlaid with a cosine curve and containing parameters for cycle length, cycle amplitude and phase) was used in a multivariate analysis of changes in serum progesterone, serum aldosterone, serum osmolality and urinary potassium. A highly significant monthly cycle in progesterone and aldosterone and a significant cycle in the urinary excretion rate of potassium were found. The individual rates of increase in serum progesterone and serum aldosterone were different and uncorrelated, and in the monthly cycle there was a phase difference between them. We think that the changes in serum progesterone and serum aldosterone are controlled by different stimuli and that the control of the cyclical component originates in the maternal neuro-endocrine system.