Most mares can safely produce foals into their mid-to-late teens, but fertility drops steadily with age, and the risks climb sharply after 15 to 16. There is no single cutoff age that applies to every mare. A healthy 20-year-old with a clean uterus may conceive and deliver without incident, while a hard-used 14-year-old with uterine scarring may struggle. The real answer depends less on the number on her registration papers and more on her individual reproductive health.
How Fertility Changes With Age
A mare’s fertility begins declining earlier than most owners expect. Data from Japanese Thoroughbred broodmares showed that live foal birth rates gradually decreased starting from as young as 3 years old, with roughly 99% of all matings occurring between ages 3 and 20. Peak breeding activity clusters around age 8. The decline is gentle at first, then steepens noticeably in the mid-teens.
The eggs themselves are a major part of the problem. Older mares still produce follicles, sometimes even more of them than younger mares, but those follicles tend to be smaller with reduced blood flow and lower antioxidant protection. The result is poorer egg quality, which translates directly into lower conception rates, higher early embryo loss, and more pregnancies that fail in the first few weeks.
By the time a mare reaches her early 20s, getting pregnant at all becomes genuinely difficult. In clinical oocyte transfer programs, pregnancy rates average around 50% for mares under 15 but drop to roughly 16% for older mares. That gap reflects the steep decline in viable eggs, not just uterine health.
Physical Risks for Older Pregnant Mares
Even when an older mare does conceive and carry to term, the pregnancy and delivery carry elevated risks. The most serious is uterine artery rupture, a potentially fatal complication that occurs during or shortly after foaling. In a large postmortem study, 78% of mares diagnosed with periparturient arterial rupture were 15 years of age or older. The uterine artery is the vessel most commonly affected, and older mares are disproportionately represented. While the overall incidence is low (about 1.5% of equine necropsy cases in the study), it is a risk that essentially does not exist in younger mares.
Difficult births also become more common. When older mares require a cesarean section due to dystocia, their outcomes are worse. Mares 16 and older who underwent cesarean delivery had a subsequent foaling rate of only 31%, compared to better odds in younger mares. The longer dystocia lasted before surgical intervention, the worse the outcome for both mare and foal.
Older mares are also more prone to placentitis (infection or inflammation of the placenta), inadequate uterine contractions during labor, and slower recovery after foaling. The cumulative wear on the uterus from previous pregnancies compounds these risks.
How Age Affects the Foal
The dam’s age has measurable effects on foal development. Mares under 7 and over 11 tend to produce lighter foals at birth than mares in their prime (roughly 7 to 11 years old), and those weight differences persist well beyond the neonatal period.
Research on Thoroughbred foals from 20-year-old dams is particularly striking. Compared to foals from 8-year-old dams, these foals weighed about 1.6 kg less at 22 days old, a gap that widened to 18.3 kg by 510 days of age. They were also significantly shorter and had smaller cannon bone circumference at multiple measurement points through their first year and a half. These aren’t catastrophic differences, but they suggest that older dams provide less nutritional support during pregnancy and possibly through milk production as well.
What a Pre-Breeding Exam Looks For
Age alone shouldn’t be the deciding factor. A veterinary breeding soundness exam is the best way to determine whether an individual mare is a reasonable candidate for pregnancy, especially once she’s past 15. The exam typically includes a physical evaluation of the external genitalia and mammary glands, followed by transrectal palpation and ultrasound of the uterus and ovaries.
Three diagnostic tests provide the clearest picture of uterine health. An endometrial culture identifies bacteria or other pathogens living in the uterus. An endometrial cytology examines uterine cells under a microscope for signs of inflammation. And an endometrial biopsy, where a small tissue sample is taken from the uterine lining, reveals scarring, fibrosis, glandular changes, and lymphatic damage. The biopsy is graded on the Kenney-Doig scale, the international standard for classifying uterine health in mares. The categories predict the likelihood of carrying a foal to term: a mare with a Category I uterus has excellent prospects, while a Category III score suggests the uterine lining is too damaged to sustain a pregnancy. Categories IIA and IIB fall in the middle, with roughly a 50% expected foaling rate.
Not every mare needs a biopsy, but culture and cytology are recommended for any mare being evaluated for breeding, regardless of age. For mares over 15, the biopsy becomes especially valuable because it can reveal damage that ultrasound alone might miss.
Hormonal Support During Pregnancy
Older mares are more likely to need progesterone supplementation to maintain early pregnancy, since their own hormone production may not rise quickly or consistently enough to support the developing embryo. The two most common approaches are a daily oral synthetic progestin or injectable progesterone in oil. Treatment typically begins around day 18 of pregnancy and continues through at least day 45, when the placenta takes over progesterone production. Your veterinarian will monitor hormone levels and adjust timing based on how the pregnancy progresses.
Embryo Transfer as an Alternative
For mares whose genetics are valuable but whose bodies may not handle pregnancy safely, embryo transfer offers a workaround. The older mare is bred and the resulting embryo is collected, then implanted into a younger, healthier recipient mare who carries the foal to term. This sidesteps the physical risks of pregnancy and delivery for the aging mare entirely.
The catch is that embryo recovery rates also decline with age. Mares in their prime have embryo recovery rates above 70%, while mares over 16 drop to around 55%. Combined with the lower egg quality that comes with age, it may take multiple breeding cycles to obtain a viable embryo. Still, for an older mare with a poor uterine biopsy score or a history of complications, embryo transfer is often the safest path to producing one more foal.
Practical Age Guidelines
While every mare is different, some general patterns hold. Mares between 3 and 15 are in their functional reproductive window, with the best outcomes clustered between 7 and 11. From 15 to 20, fertility declines noticeably and pregnancy risks increase, but many mares in this range still produce healthy foals with appropriate veterinary management. Beyond 20, natural conception becomes quite difficult, pregnancy complications rise substantially, and most veterinarians will recommend a thorough workup before giving the green light.
A mare is “too old” not at a specific birthday but at the point where her uterine health, egg quality, and physical condition make pregnancy unlikely to succeed or unsafe to attempt. A breeding soundness exam, particularly the endometrial biopsy, is the most reliable way to make that call for your individual mare rather than relying on age alone.