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

Getting a mare in foal sounds straightforward until it isn't. Equine reproduction has a surprisingly high failure rate at every stage, from the moment of ovulation to the day a foal 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 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. Mares with chronic degenerative changes in the endometrium show measurably poorer placental development later in gestation than mares with healthy uterine tissue [2].

Endometrial fibrosis

The most common finding in reproductively compromised mares is endometrial fibrosis. The precise term, coined by Kenney, is endometrosis, not to be confused with endometriosis, the very different condition seen in women. Periglandular fibrosis encases the uterine glands, reducing their secretory capacity. Kenney and Doig (1986) found that a majority of clinically healthy mares had some degree of endometrial fibrosis on biopsy, a proportion that climbs sharply with age [1]. Biopsy grading remains the most reliable way to assess the severity of this damage [1].

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, and the resulting environment is hostile to an embryo. This is treatable, and catching it early matters more than almost anything else in this section.

Anatomy

Conformation affects susceptibility to uterine contamination. Mares with poor perineal conformation, particularly common in Thoroughbreds, can aspirate air into the vagina with each step, introducing environmental contamination to the reproductive tract [3]. 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 [4].

The embryo dies after implantation

Early embryonic loss is common, and the published numbers vary a lot depending on which mares you're looking at. Ball's frequently cited 1988 review put losses through day 14 at under 10% in young, reproductively normal mares, climbing past 60% in aged, subfertile ones [5]. A field study of nearly 1,400 Thoroughbred mares in Newmarket found roughly 10% loss between day 15 and day 35 alone, even in a well-managed, actively monitored population [6]. Most of this happens before day 40, which is why a rectal palpation at first pregnancy check doesn't guarantee a live foal nine months later.

Loss becomes less frequent after that point but doesn't disappear. A 2022 UK study tracking 4,439 Thoroughbred pregnancies found that 7.3% of mares confirmed in foal at day 70 still failed to produce a live foal, and just over half of those losses were classed as abortion rather than stillbirth or other causes [7]. Rhinopneumonitis, caused by equine herpesvirus types 1 and 4 (EHV-1 and EHV-4), gets most of the attention here, and rightly so: EHV-1 can trigger abortion storms that wipe out several pregnancies on one stud within weeks. But in that same UK study, only around 9% of losses past day 70 were linked to EHV [7]. Most had other causes entirely, twinning, umbilical cord accidents, and placentitis among them. Vaccination still matters. If your mares aren't on a protocol with boosters at months 5, 7, and 9 of pregnancy, that needs fixing. It just isn't the only box to tick.

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 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's usually a combination of factors, some fixed and some not. Knowing the difference is most of the clinical work.