Below, please find links to all of the clinical trials involving reproduction. The studies include a multitude of information, including (but not limited to) the study’s purpose, benefits for participating, and financial incentive information. If you have any questions, please contact the individual outlined at the end of each trial summary.

Please visit the Equine Reproduction service webpage at the Veterinary Medical Teaching Hospital (VMTH) if you would like to learn more about the amazing things that our veterinarians can do for you and your animal.


Fertility: Understanding subfertility in older maiden mares

Title: Endometrial biopsy-related changes in older maiden mares

Purpose: Older, maiden mares have a reputation for being subfertile. It has been shown that maiden mares older than 13 years have reduced fertility compared to mares that have had previous foals. The specific reasons for the reduced ability to become pregnant and maintain a pregnancy are unknown. This study aims to compare biopsies from the uterus of older maiden mares with mares of comparable age to look for detrimental changes that can explain the drop in fertility. Knowing specific causes of subfertility allows targeted treatment to help increase fertility.

Contact: Dr. Bruce Christensen, DVM, MS, DACT at or (530) 752-0292

Participation Requirements: Mares that are older than 12 years old and have either:

  • Not had a pregnancy in the last 6 years; or,
  • Have had consecutive pregnancies for 5 of the previous 6 years, including the most recent year.

Initial Evaluation for Participation: None.

Procedures: The trial procedures include the following:

  • A transrectal ultrasound evaluation will be performed to determien the stage of estrous cycle of your mare.
  • Your mare’s tail will be wrapped and tied to the side. The vulvar area of your mare will be washed with a mild detergent and dried off.
  • The veterinarian will put on a sterile sleeve and insert a sterile biopsy instrument (70 cm long, alligator-type biopsy instrument with 20 x 43 mm sample basket) that is guided through the cervix with the gloved hand. Once in the uterus, the biopsy instrument is opened. The hand is withdrawn from the vagina and inserted into the rectum. The open instrument is palpated per rectum and the endometrium is gently pressed into the open instrument. The instrument is closed and withdrawn from the uterus.
  • The mare’s vulvar area is cleaned with water.
  • If your mare was determined to be in diestrus (out of heat), an injection of prostaglandin will be given to bring the mare into heat and avoid an inadvertent uterine infection.

NOTE: Mares do not usually require sedation for the procedure; however, if your mare is uncomfortable standing still in the equine stocks or nervous, we will sedate your mare with xylazine (a short-acting sedative given intravenously). Additionally, mares do not usually require pain medications for this procedure because the inner lining of the equine uterus does not contain sensory innervation.

Benefits: We cannot promise any benefits to your mare or other animals from your taking part in this clinical trial; however, possible benefits include specific knowledge about any uterine disease in your particular mare. This will be potentially be very useful in formulating a breeding management plan for your mare, should you decide to have her bred. Your mare’s participation will also help us better understand age-related and parity-related (number of foals a mare has in her life) changes in a mare’s uterus that can affect the ability to maintain a pregnancy.

Owner Responsibilities: If you allow your mare to participate in this study and desire to have the results for your own personal use, you will be responsible to pay for a board-certified pathologist to evaluate the biopsy. No charge will be assessed for taking the biopsy, only for the interpretation of the results. If you do not desire to have the results for your own use, you will not have any financial responsibility; however, the results will not be given to you.

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