Indirect pregnancy diagnosis in the mare: experiences with the determination of estrone sulfate in blood and urine.

Pregnancy diagnosis in the mare is routinely performed after day 18-20 by rectal palpation usually aided by ultrasonography. In comparison, indirect methods for pregnancy diagnosis are rarely applied with the detection of equine chorionic gonadotrophin (eCG; PMSG) being the most popular test system. However, alternatively to the detection of eCG, the radioimmunological or enzymeimmunological determination of pregnancy-associated oestrogens can also be used for hormonal pregnancy diagnosis in the horse. Based on the methods and data established by Gentz (1994), pregnancy diagnosis in the mare has been performed routinely since about seven years in our laboratory by the determination of estrone sulfate concentration in blood plasma or serum. The experiences made so far show that an unequivocal diagnosis of pregnancy can be performed from the onset of placental oestrogen biosynthesis at around days 80-90 of gestation until the steep decline during parturition and, different to the detection of eCG, no false positives were obtained. In the period between days 40-80 after breeding, pregnancy-associated oestrogens of ovarian origin can be used for pregnancy diagnosis. However, estrone sulfate concentrations of pregnant and nonpregnant mares overlap in this period. Values higher than 4.5 nmol/l (1.6 ng/ml) are indicative for intact gestation, whereas pregnancy can be excluded if concentrations are lower than 2.4 nmol/l (0.8 ng/ml). In the case of overlapping values, detection of eCG can be applied additionally to verify conception. The same method seems also applicable for the determination of urinary estrone sulfate which would allow replacement of overcome chemical assays. Preliminary results suggest that determination of urinary estrone sulfate concentrations enables pregnancy diagnosis throughout the whole period of fetoplacental oestrogen biosynthesis.