Effect of ovariectomy on pregnancy in mares.

The study investigates the impacts of ovariectomy (removal of ovaries) on pregnancy in mares. The experiment conducted on pony mares reveals that ovariectomy during early stages of gestation leads to abortion or fetal resorption, while the procedure does not affect pregnancies if performed on later stages.

Methods

• The research was carried out on pony mares which were bilaterally ovariectomized at various stages of pregnancy – Day 25 to Day 210.
• The response of the mares to the operations at different time points was observed and recorded.

Findings

• All 14 mares that underwent ovariectomy between Days 25 and 45 of pregnancy aborted or experienced fetal resorption within 2 to 6 days post-operation.
• Abortions were observed in 9 of the 20 mares that were ovariectomized between Days 50 and 70, albeit after a longer interval of 10 to 15 days.
• Interestingly, when ovariectomy was conducted on Day 140 or 210, the mares were able to carry their foals to a normal term.

Implications

• The study indicates that the removal of ovaries could have a hazardous impact on pregnancies at early stages, characterized by a loss of uterine tone and the uterine bulge.
• Progestagen concentrations dropped below 2 ng/ml after the mares underwent ovariectomy. However, this drop did not appear to have any effect on the ability of the mares to maintain pregnancy if ovariectomy was performed later in gestation.

Experiments with Progesterone

• Mares that were ovariectomized on Day 25 and given daily injections of 100 mg progesterone for 10 or 20 days managed to sustain their pregnancies during the treatment. However, 4 to 6 days post-treatment, these mares showed loss of uterine tone and the vanishing of the fetal bulge.
• Moreover, whether pregnant or not, both ovariectomized and intact seasonally anoestrous mares showed similar changes in uterine tone, length, and thickness – typical of early pregnancy – when treated daily with 50 or 100 mg progesterone for 8 weeks.

Conclusion

• The mean gestation lengths of all mares whose pregnancies were not interrupted by ovariectomy were not significantly different from that of a group of control mares. This signifies that ovariectomy does not influence the duration of gestation if the pregnancy was not affected by it.