Prediction Values for the Influence of Fetal Sex on Plasma Progesterone Concentration in Crioulo Breed Mares: A Preliminary Investigation

In equine reproduction, determining foetal sex is of economic and strategic importance, but currently available methods are often invasive, costly, or require specialised expertise. This study aimed to assess whether plasma progesterone concentrations could serve as a predictive tool for foetal sex determination in Criollo mares between the 4th and 8th months of gestation and to determine whether these measurements could identify foetal sex. Blood samples were collected at 30-day intervals from 17 Crioula mares between 114 days and 240 days of gestation. Maternal plasma progesterone concentrations were determined by radioimmunoassay and the sex of the foals was confirmed at birth. Analysis of Variance (ANOVA) was made to verify the variation in maternal progesterone concentrations according to foetal sex and month of gestation. In mares carrying male foetuses (n = 7), there was a significant difference in progesterone concentrations between months 4 and 8, as well as between months 7 and 8. In mares carrying male foetuses (n = 7), progesterone concentrations were higher (p = 0.028) during the 4th month of gestation and lower at the 8th month (p = 0.020), compared to the values in mares with female foetuses (n = 10). In the 8th month of pregnancy, the sensitivity and specificity of progesterone concentrations for the predicted sex were 80% and 100%, respectively. A limit value of progesterone (12.45 ng/mL) was established through the ROC (receiver operating characteristic) Curve. Prediction values were 78.8% and 100% for males and females, respectively. Detection rates were 100% and 80% for males and females, respectively. The diagnostic accuracy for both sexes was 88.2%. It is concluded that foetal sex influences plasma concentrations of progesterone in pregnant mares.