An oestrogen conjugate enzyme immunoassay for monitoring pregnancy in the mare: limitations of the assay between days 40 and 70 of gestation.

This research developed an enzyme immunoassay to monitor pregnancy in mares using oestrogen measurements in the bloodstream, but found it less reliable between days 40 and 70 of gestation.

Research Methodology

• The researchers developed an enzyme immunoassay, which is a biochemical test designed to measure a specific antigen in the blood.
• This specific immunoassay was aimed at measuring levels of conjugated oestrogens, particularly oestrone-3-glucuronide (E1G), in the plasma of pregnant mares. Conjugated oestrogens are a form of the hormone that has been modified in the body and then excreted.
• To achieve this, they produced an antibody in rabbits, by inducing an immune response using E1G bonded to bovine serum albumin.
• They also created an enzyme conjugate by bonding E1G to horseradish peroxidase, a common enzyme used in scientific tests.

Key Findings

• The researchers observed that E1G levels in the plasma increased significantly between days 35 and 40 of the mare’s gestation period.
• These levels dropped slightly by day 45, remained more or less constant until around day 70, and then rose sharply again.
• Interesting, the death of the fetus prior to day 35 did not affect E1G levels.
• But if the fetus died after day 35, in conjunction with the endotoxin-induced regression of the corpus luteum (CL) — the structure that develops in an ovary after an ovum has been discharged but degenerates after a few days unless pregnancy has begun — it resulted in a decrease in plasma E1G levels back to non-pregnant values within 3-4 days.
• They also noted that the E1G levels did not immediately drop following the death of a fetus without CL regression after day 35, and the existing levels could sometimes be maintained for up to 10-14 days.

Conclusion

• These findings suggest that between days 35 and 70 of a mare’s pregnancy, plasma E1G concentrations are not directly linked to the fetus’s viability. Rather, they seem to reflect the increased oestrogen secretion by the CL under the influence of the hormone chorionic gonadotropin.
• After day 70, however, the E1G concentrations appear to directly reflect the production of oestrogen by the developing feto-placental unit.