What is the gestation period in horses?

The average length of pregnancy or gestation length is 340 days. In ponies it is slightly shorter at 333 days. However this can be a variable time period and a normal healthy foal may be born anywhere between 325-365 days gestation.
The length of pregnancy can be influenced by various factors including nutrition, as well-fed mares will often have shorter pregnancies. Also, the time of year can have an influence on how long a mare will remain pregnant, and mares that are carrying foals through the winter months often have a longer gestation period.

How is pregnancy in horses diagnosed?

The traditional method for diagnosis is rectal palpation where the foetus or foal is felt for manually. Ultrasound examination has become increasingly used as the foetus can be visualised on a screen and often the pregnancy can be detected much earlier than with manual palpation. Pregnancies can be detected as early as 14 days after ovulation. However progressive scans are required to ensure that the pregnancy has persisted and is healthy.

Ultrasound examination involves insertion of a probe via the rectum that relays an image back to a screen, which can then be viewed and stored. Although this is the most useful and most common method of diagnosis there are some inherent risks to the mare and the vet during this procedure. Rectal tears are rare, but potentially catastrophic events for the mare. Restraining the mare in stocks during the examination can reduce the risk of this happening. Light sedation or gut muscle relaxants can also be used if a mare is particularly anxious. The use of stocks will also protect the vet from being kicked. There are obviously size limitations for this examination to be carried out safely and small breeds or fractious horses are unsuitable candidates.

There are two types of blood test, which can also be used to diagnose pregnancy depending on the stage of the expected pregnancy. The first looks for the presence of a hormone called equine chorionic gonadotrophin (eCG). Equine chorionic gonadotrophin is produced by the placenta during pregnancy between days 45 and 90. After 100 days of pregnancy a different blood test is used to look for the presence of a hormone called oestrone sulphate, which is produced by the foetus.

If you wish to discuss breeding or pregnancy testing in your mare Central Equine Vets will be able to discuss with you in detail the different methods of diagnosis and which one(s) are most appropriate for your mare.

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