Possible circadian periodicity of foetal prolactin secretion late in gestation.

Prolactin levels were determined in samples of umbilical venous blood at vaginal delivery after 38-42 weeks of uncomplicated gestation. The mean level of prolactin for all samples was 328.4 +/- 7.4 (SE) ng/ml. The levels did not vary significantly during these weeks of gestation, nor correlate with the duration of labour. The means of levels combined every 2 h varied significantly throughout the 24 h period with a maximum value of 460.3 +/- 85.1 ng/ml at 2-4 a.m. and a minimum value of 241.4 +/- 27.2 ng/ml at 6-8 a.m. and remained relatively constant between 2-4 and 10-12 p.m. These results suggest that a circadian periodicity may exist in the secretion of prolactin by the human foetus, and that this is not affected by labour and delivery.