Why can't I get my mare to go in foal?

Getting a mare in foal sounds straightforward until it isn't. The reality is that equine reproduction has a surprisingly high failure rate at every stage — from the moment of ovulation to the point when a foal actually arrives. Understanding where in the process things break down changes how you approach the problem, and what you can realistically fix.

The egg never gets fertilised

The most basic failure is no fertilisation at all. That can come from the mare's side or the stallion's.

On the mare's side

Timing is everything. Mares are seasonally polyoestrous, meaning they cycle reliably from May to September but often show irregular, anovulatory cycles in late winter and early spring. Breeding outside the natural season dramatically reduces the chance of a viable ovulation. If you're working with a mare that "never catches," ask first when she's actually being bred.

Egg quality declines with age. Older mares produce oocytes that are more likely to have chromosomal abnormalities or reduced developmental competence. This isn't something you can see on an ultrasound — it only becomes apparent in poor embryo development rates.

On the stallion's side

Semen quality varies more than most owners expect. Fresh semen used at the right moment gives the best outcome. Chilled semen, collected and transported before use, loses progressive motility over time. Frozen-thawed semen has even lower post-thaw survival rates and an extremely narrow insemination window. If you're working with frozen semen from a stallion with a poor freeze, the odds are already stacked against you before the mare is even involved.

The fertilised egg doesn't implant

An embryo reaches the uterus around day 6 after ovulation. What happens next depends almost entirely on the health of the endometrium — the uterine lining. This is where most chronic infertility problems live.

The endometrium does more than provide a surface for implantation. It produces uterine milk, the secretion that sustains the embryo in the days before placentation begins. A damaged endometrium cannot provide adequate nutrition, and the embryo fails silently.

Endometrial fibrosis

The most common finding in reproductively compromised mares is endometrial fibrosis, also called endometriosis in horses. Periglandular fibrosis encases the uterine glands, reducing their secretory capacity. Kenney and Doig (1986) found that 64% of clinically healthy mares had some degree of endometrial fibrosis on biopsy [1]. That number is worth pausing on: nearly two-thirds of apparently normal mares, and that proportion rises sharply with age. Biopsy grading remains the most reliable way to assess the severity of this damage [2].

The problem with fibrosis is that it doesn't reverse. Once those glands are encased in scar tissue, they're gone. Management focuses on optimising what remains, not on restoring what's lost.

Endometritis

Acute and chronic inflammation of the uterus is another major cause of implantation failure. Some mares have impaired uterine clearance — they can't efficiently evacuate fluid and inflammatory debris after breeding. The resulting environment is hostile to an embryo. This is treatable, and catching it early matters.

Anatomy

Conformation affects susceptibility to uterine contamination. Mares with a poor perineal conformation — particularly common in Thoroughbreds — can aspirate air into the vagina with each step, introducing environmental contamination to the reproductive tract. Saddlebreds, in my clinical experience, are disproportionately represented in chronic fungal endometritis cases. These are breed-level tendencies, not universal rules, but they shift the differential when a mare has a history of persistent infection [3,4].

The embryo dies after implantation

Early embryonic death occurs in 4 to 20% of mares — the range reflects real differences in population and methodology across studies, not uncertainty about whether the problem exists [1]. Most of these losses happen before day 40, which is why rectal palpation findings at first pregnancy check don't guarantee a live foal.

After day 40, abortion becomes less common but still occurs in approximately 7 to 8% of pregnancies [1]. The single most important infectious cause of abortion in mares is rhinopneumonitis, caused by equine herpesvirus types 1 and 4 (EHV-1 and EHV-4). EHV-1 in particular can cause abortion storms in naive groups. Vaccination reduces the risk, though it does not eliminate it entirely. If your mares are not on an EHV vaccination protocol that includes boosters at months 5, 7, and 9 of pregnancy, that needs to change.

Where to start when nothing is working

Before spending significant money on repeat breeding attempts, a uterine biopsy gives you the clearest picture of prognosis. A Kenney grade I biopsy carries a good prognosis for carrying a foal to term; grade III carries a poor one. That information is worth having early rather than after several more failed cycles.

Beyond the biopsy: stallion semen evaluation if using AI, a uterine culture and cytology to rule out active endometritis, and an honest look at the breeding calendar. Many mares that are labelled as infertile were simply bred at the wrong time of year, with the wrong semen type, or with no ultrasonographic monitoring of ovulation.

Reproductive failure in mares is rarely one thing. It is usually a combination of factors, some of which are fixed and some of which are not. Knowing the difference is most of the clinical work.