Laser Ablation of Endometrial Cysts Can Improve Mare Pregnancy Rates 

Research has shown that 1-22% of mares in the general population and more than 55% of subfertile mares will have endometrial cysts. Why is this important? 

As Karen Wolfsdorf, DVM, DACT, of Hagyard Equine Medical Institute, in Lexington, Kentucky, explained during a presentation at the 2025 British Equine Veterinary Association Congress, endometrial cysts are signs of poor uterine clearance and blocked lymphatics. These structures protrude into the lumen of the uterus and can be very difficult to differentiate from a 14-day pregnancy or twins on ultrasound. In addition, they might be associated with early embryonic loss. 

“The embryo needs to be able to move around the uterus for maternal recognition of pregnancy to occur,” she said. “Unfortunately, endometrial cysts, when large or numerous, can obstruct the uterine lumenand inhibit mobility. Because these embryos tend to fix next to the cyst, as that embryonic vesicle grows, nutrient and oxygen flow can be inhibited between the endometrium, disturbing the normal growth of the embryonic vesicle.” 

Background and Case Population 

While veterinarians can perform a variety of procedures to remove endometrial cysts, Wolfsdorf typically uses laser ablation on smaller or multiple cysts. Anecdotally, referring clinicians have reported good success rates with getting affected mares in foal after laser removal of cysts. 

“I thought, let’s put some scientific basis behind the fact that they are getting in foal,” she said. 

So Wolfsdorf and her team at Hagyard performed a retrospective clinical study to determine if endometrial cyst removal via hysteroscopic laser improves pregnancy rates in barren or early embryonic loss mares. The case population included 240 mares from one referral practice undergoing endometrial cyst ablations over a 10-year period. All mares had been bred at least one estrous cycle without getting pregnant, had been barren from previous years, or had early embryonic loss. The team looked at each mare’s: 

• Age. 

• Number of cysts. 

• Hysteroscopic observations (e.g., any debris or mucus, how hyperemic the epithelium of the uterus looked). 

• Post-procedure treatment (the mares usually received two to three uterine lavages to get rid of debris and inflammation). 

• 14-day pregnancy rates. 

Study Findings 

Following laser ablation of cysts, most mares (62%) got pregnant, with a majority (53%) getting pregnant within one or two cycles, said Wolfsdorf. 

Age did not affect whether a mare got pregnant, “but we have to consider that most of the mares that came to us were older (average age 17.5), so that could have something to do with it,” Wolfsdorf added. 

She reported no significant relationship between the number of cysts ablated and pregnancy outcome but noted a trend: Mares that had fewer cysts had higher pregnancy rates compared to mares with a greater number of cysts. 

“In conclusion, laser ablation of endometrial cysts improves 14-day pregnancy rates on subsequent estrous cycles,” Wolfsdorf said. “Pregnancy rate was not impacted by age or severity of cysts, keeping in mind that this was an older population that we were looking at.” 

One limitation of this study is the lack of true controls. “The mares essentially served as their own controls because they had been bred numerous times on previous cycles and hadn’t gotten in foal with presumably the same pathology that was there,” she said. 

Based on these findings, “It makes me feel a lot more comfortable saying yes, we can perform this procedure, and we should have better pregnancy rates associated with it,” said Wolfsdorf. “But we still need to go back and recognize that it is a sign of an underlying problem (poor uterine clearance associated with mare age and multiple pregnancies), and we can still have recurrence of these endometrial cysts.”  

For this reason, she recommended performing laser ablation just prior to the breeding season to minimize the chances of cyst recurrence that season. She also advised practitioners to relay to clients that removing the cysts can help get their mares in foal, but they’re probably going to come back at some stage. 

Related Reading

• Factors Affecting Pregnancy Rates in Mares Bred With Frozen Semen
• Strategies for Reducing Early Embryonic Loss in Mares
• Managing Pregnant Mares From Conception to Parturition

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