Is it rare for a horse to have a miscarriage?

Answer 1

Abortions in mares are not rare. Like humans, most unsuccessful pregnancies end early. The period of EED (early embryonic death) can occur

anywhere up to 34 days without effectively compromising the ability of the mare to become pregnant in a single breeding season.

Any EED that occurs after 35 days results in the mare not returning to estrus for several months which generally results in a late or no foal. Breeders who want to produce early foals for sales or futurities stand to take a considerable economic "hit" by producing a late foal.

A primary cause of abortions in mares is the presence of twins. Since horses rarely succeed in producing live twin foals the industry standard is to destroy

the smaller conceptus 14-16 days post ovulation. Breeds that tend to double ovulate (Thoroughbreds are a good example) are usually monitored closely

for twins because of the financial loss represented by middle or late term aborted twins.

Other causes of miscarriage/abortion can be disease, inflammation, genetic defect, illness, ingestion of plant and other toxins, the use of products that contain organophosphates (some wormers), inappropriate vaccination during organogenesis or placental insufficiency to name a few.

While the litany of reasons for miscarriage are many, by and large most mares successfully pregnant at 35 days (carrying a single fetus) is quite high.