Endocrine changes, fetal growth, and uterine artery hemodynamics after chronic estrogen suppression during the last trimester of equine pregnancy.

The research article investigates the effects of suppressing estrogen production during the final trimester of horse pregnancies. Specifically, the study analyzed whether lowered levels of estrogen affected the blood flow in the uterine artery, the development of the fetus and the placenta, and the health of the newborn foal. The study found that lower estrogen levels had no effect on most of these factors, except that foals born to mares treated with an estrogen inhibitor had lower birth weights.

Approach

• The study involved twelve pregnant mares, which were split into a control group and a treatment group. The latter was given letrozole, a drug that inhibits the production of estrogen, from the 240th day of their pregnancies until they gave birth.
• Throughout the experiment, weekly serum samples were collected from the mares to track their levels of various hormones, including testosterone, dehydroepiandrosterone sulfate, estradiol, estrone sulfate, progestins, and prostaglandin F2α metabolite.
• The researchers also conducted biweekly ultrasonic examinations to measure aspects of the uterine artery’s functioning, the growth of the fetus (measured via the size of its eye), and the combined thickness of the uterus and placenta — an indicator of placental health.
• At birth, the researchers recorded the length of gestation, the foal’s weight, and its viability at birth.

Findings

• The use of letrozole resulted in a notable reduction of estrogen synthesis — about 90% compared to the levels in the control group.
• This significant decrease in estrogen levels did not affect uterine artery hemodynamics, normal placental development, duration of pregnancy, or neonatal viability. In other words, the horse’s bodies adequately compensated for the lower estrogen levels when it came to essentially all aspects of pregnancy and giving birth.
• The only difference observed was in relation to birth weight. Foals born to mares that had received letrozole had lower birth weights than those born to the control group. This suggests that estrogens play a role in fetal growth that isn’t necessarily tied to regulating blood flow to the uterus.

Significance

• The research provides insights into the role of estrogens in equine pregnancies, a subject that had not been fully understood prior to this study.
• This work specifically contributes to our understanding of how estrogen influences fetal growth and might be useful in the prevention of late pregnancy losses, often linked to low estrogen levels.
• More broadly, this experiment opens up potential areas for further research and may have implications not only for veterinary medicine, but also for human reproductive health.