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Equine Health Resource

Sterile Glass Marble Placed in Uterus



A small glass marble (usually 35mm in diameter) can be placed in a mare’s uterus to suppress behavioral estrus. Other materials may be used too – stainless steel and plastic spheres have been shown to have similar effect. It is essentially an IUD (intra-uterine device).

The presence of the ball in the uterus causes the corpus luteum (the ovarian structure that produces progesterone) to remain after ovulation. This keeps the level of progesterone elevated enough to suppress estrus for a period of time.

After sterilization in an autoclave, the ball is inserted through the cervix into uterus within 24 hours after ovulation, while the mare is still in heat. Ultrasound and palpation is often used to ensure proper timing. Generally, mares tolerate the balls well. They are occasionally lost and are easily removed. Timing of placement is key.

This Treatment Might be used for a horse exhibiting these signsRelated Observations

Related DiagnosesThis Treatment Might Be Used for these Diagnoses

Consider Potential Side Effects & Complications

The marble may be lost. If the marble is too small, it can be easily expelled.

Sometimes people forget to remove the balls. Vets occasionally find multiple IUD devices in the uterus of mares. In some cases, this can result in uterine injury. Occasionally, the glass balls may break or shatter (when they come into contact), resulting in uterine injury and development of scar tissue.

This treatment only works in a percentage of mares. In my experience, about 50% of mares have a prolonged period where they do not show heat.

If you are not the first owner of the mare or otherwise do not know her complete reproductive history, talk to your vet about examining the mare to confirm that no marbles are already in her uterus.

Consider Reasons Not To Use This Treatment

Not usually used in conjunction with oral hormone therapy. Not used in mares with existing uterine problems like infection.

Is It working? Timeframe for effect

Immediate. A mare should not show estrus signs once the marble has been placed. This should last a variable period of time; several months at minimum.

Questions To Ask My Vet

  • Should we confirm that no marbles are already in the mare?
  • What is the success rate, in your experience?
  • Should we try this treatment before we use oral progestin (Regumate)?
  • What has your experience been with the placement of a marble "IUD"?
  • When will the marble be removed?
  • What are the dangers in using this treatment?

Helpful Outside ResourcesCredible Equine Health Information on the Internet

Author: Doug Thal DVM Dipl. ABVP


Vanderwall, DK. Is it time to retire the use of intrauterine glass balls for estrus suppression in mares. JAVMA Vol. 247, No. 4 August 15, 2015


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