In this study, effects of altrenogest treatment (0.088 mg/kg daily) given to mares during late gestation until parturition on the time and the process of foaling, neonatal adaptation and postnatal development were analysed. The number of animals was 6 in the treatment group and 7 in the control group. Gestational length tended to be shorter in mares given altrenogest. Birth weight of the foals and weight of the placenta did not differ between groups. The second stage of parturition was prolonged in the altrenogest-treated mares (p<0.05). Foals born to altrenogest-treated mares had a significantly lower respiratory rate than control foals during the first 30 minutes of life (p<0.05). At no time differences in heart rate and body temperature were found between groups. In foals of altrenogest treated mares, venous plasma pH was significantly higher than in control foals at 15 and 30 minutes after birth (p<0.05). Base excess in foals of altrenogest treated mares was significantly higher than in control foals at 45 minutes and up to 12 hours after birth (p<0.05). There were significantly more problems in the perinatal period (3/6) in foals born after altrenogest treatment to their dams than in control foals (0/7; p<0.05). In conclusion, treatment with altrenogest did not prevent parturition and its effectiveness to prevent abortion or preterm foalings in mares with disturbed pregnancies should be doubted. In addition, altrenogest treatment of mares affected adaptation of the foals to the extrauterine environment.

Altrenogest treatment of pregnant mares prolonged labour had no major effects on adrenocortical function in foals. A reduced neutrophil/lymphocyte ratio in these foals may suggest either immunomodulatory effects of altrenogest or dysmaturity of the foals.

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