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This study aimed to understand the physiological mechanisms regulating parturition and to identify potential biomarkers to predict onset of birth. Additionally, we compared hormone profiles between cows with shorter and longer gestation lengths. Twenty-eight days before due date until 3d postpartum, cows (n = 18) were blood sampled daily. Circulating concentrations were measured for progesterone (P4) and estradiol (E2) by RIA, testosterone, prostaglandin F2α metabolite (PGFM), cortisol, pregnancy-specific protein B (PSPB) by ELISA and lactate concentrations by colorimetric assay. At end of gestation, P4 decreased from d-14 to d-4 (from 3.6 to 1.4 ng/mL), most likely from rapid loss of placental P4 production (64% of decline in 24 h). A second rapid decrease in P4 to undetectable concentrations was observed from d-2 to parturition (from 1.4 to 0.1 ng/ml; most likely luteal origin) corresponding to increase in PGFM from d-2 to parturition (249.7 to 2868.4 pg/mL). Estradiol and PSPB increased ~8-fold from ~13d before parturition with acute rise in E2 but not PSPB (45% vs 13% in first 24 h). Testosterone decreased slightly during the same period. Cortisol and lactate increased only at calving. Comparison of cows with shorter vs longer gestation, when data were normalized to parturition day, a difference was detected in circulating E2 and PGFM patterns, but not P4 and PSPB. Thus, the first significant hormonal changes associated with parturition begin at d-14 with E2 and PSPB as two clear biomarkers of impending parturition. Cows with shorter and longer gestation had hormonal differences indicative of identifiable earlier placental maturation.
Monteiro et al. (Mon,) studied this question.