The typical gestation period for horses ranges from 340 to 342 days. During this time, foals undergo extensive development of their organ and skeletal systems, ensuring their survival outside of the mare after birth.

In some cases, foals are born early, before their organs are fully developed. These “premature” and “dysmature” foals often experience respiratory problems, impaired immune function, gastrointestinal issues and metabolic problems.

Immature foals require intensive care and management to protect their health and enable them to catch up in development. It’s important for breeders to recognize premature foals quickly, so they can provide appropriate care.

Some foals are born well after their expected due date and may show signs of over-maturity. Prompt recognition of “postmature” foals is also crucial, since they are usually a sign of fescue toxicosis in the mare. Rapid intervention can help ensure the health of both the mare and foal.

Normal vs Abnormal Gestation Periods

The gestation period in mares commences on the day of insemination and ends when the mare delivers a foal.

Typically, mares will foal between 340 to 342 days of gestation, however the average gestational length can vary depending on several factors: [1]

  • Breed: Some breeds have shorter average gestational periods, such as some pony breeds, Arabians, and Friesians
  • Foal sex: Colts typically have a longer gestation period than fillies by around 1-2 days
  • Time of insemination: Mares bred earlier in the breeding season typically have longer pregnancies by up to 10 days
  • Genetic factors: There may be an association between certain bloodlines and longer or shorter gestational lengths

Abnormal Gestation Length

Gestation periods can vary in several different ways, all of which have an impact on the foal’s health at birth: [1][2]

  • Prematurity: Foals born before 320 days of gestation
  • Dysmaturity: Foals born during the normal gestation period, but with symptoms of prematurity
  • Prolonged gestation: Foals born after their expected due date, but with appropriate development for a newborn foal
  • Postmaturity: Foals born after their expected due date, with signs of continued in utero development beyond what is expected for a newborn foal
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Prematurity and Dysmaturity

Both premature and dysmature foals exhibit signs of inadequate development for life outside of the uterus.

Their treatment and management require a similarly high level of care and attention to ensure their survival and healthy development.

Causes

Immaturity in foals typically results from a disruption in the nutrient exchange between the mother and the developing fetus. Causes of prematurity and dysmaturity include: [1]

  • Life-threatening maternal illness
  • Placentitis
  • Premature placental separation
  • Twins
  • Nutritional imbalances in the mare, such as