Prematurity, Dysmaturity, and Postmaturity: Abnormal Gestational Length in Foals

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 and other systems are fully developed. These “premature” foals often experience respiratory problems, impaired immune function, gastrointestinal issues and metabolic problems. Similarly, dysmature refers to foals that experience a normal gestation period, but exhibit signs of prematurity.

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 gestation period 1 – 2 days longer than fillies
• 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

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 iodine imbalances

Inducing labour in mares can also lead to the birth of immature foals. In foals, the final stimulus to mature only occurs within the last 48 to 72 hours before delivery. ^[1]

Currently, a method for triggering this stimulus using medication has not been discovered, meaning that there is a risk of immaturity from induced parturition (birth). In general, labour should only be induced after 320 days of gestation. ^[3]

Symptoms and Diagnosis

Immature foals have a unique physical appearance at birth due to their incomplete development. Breeders may notice: ^[1][4]

• Smaller than expected foals
• Short and silky hair coat
• Domed head shape
• Lax tendons with dropped joints or elevation of the toe when standing
• Droopy or floppy ears
• Poor muscle development
• Weakness

Other symptoms displayed by immature foals include: ^[1]

• Difficulty standing
• Poor suckle reflex
• Seizures
• Rapid breathing or difficulty breathing
• Cool extremities
• Minimal urine production

Foals are usually diagnosed based on examination after birth and by calculating the length of the gestational period. Further diagnostics, such as bloodwork, may help the veterinarian determine the severity of the foal’s condition and which supportive measures the foal requires. ^[1]

Treatment and Management

Immature foals often have several compromised organ systems, which can make treatment difficult. The veterinarian must design a specific treatment plan for the affected foal, based on physical examination and diagnostic testing.

Respiratory Management

Foals that are immature often have atelectasis, a condition characterized by collapsed lungs that do not inflate fully. ^[1][5] Without proper lung inflation, foals can have difficulty breathing and develop hypoxemia, which is marked by poor oxygenation of the blood. ^[1]

Treatment for respiratory distress depends on the severity of the foal’s symptoms, but can include: ^[1][5]

• Intranasal oxygen
• Mechanical ventilation of the foal
• Glucocorticoid injections to stimulate maturation of the lungs

Foals with atelectasis should be placed in a V-pad, a trough in a V-shape that holds the foal sitting upright. ^[1] Lying flat on the ground can worsen atelectasis and prevent effective treatment. ^[1]

Some immature foals display periodic breathing, where they take long pauses in between sets of breaths. ^[1] Treatment for periodic breathing primarily involves oral caffeine or intravenous doxapram to stimulate a more regular breathing pattern. ^[1][5]

Cardiovascular Management

In utero, the foal’s cardiovascular system is uniquely adapted to facilitate blood circulation from the mother. Once the foal is born, the cardiovascular system must rapidly change to supporting its own circulation.

Premature foals typically have poor cardiovascular function, as they may not have received the appropriate developmental signals to trigger this change.

A common issue in these foals is a diminished response of the blood vessels to hormones that regulate blood pressure. ^[1] These foals may struggle to maintain adequate blood pressure, which is crucial for distributing oxygenated blood to their tissues, potentially causing hypoxia or even tissue death. ^[1]

To address the compromised cardiovascular function in premature foals, the following treatments are often employed: ^[1]

• Vasopressors: These medications are used to constrict blood vessels, thereby increasing blood pressure. Vasopressors are critical in managing foals that cannot maintain blood pressure on their own.
• Intravenous Fluids: Administering fluids intravenously helps to increase the volume of blood in the circulatory system, which can effectively raise blood pressure.

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Kidney Management

Immature foals often have poor kidney function. In the uterus, the foal’s kidneys are not required to eliminate toxins, as this function is managed by the mare. However, once the foal is born, the kidneys must become fully operational to flush toxins from the body.

Many immature foals have low urine output initially. As a result, their kidneys are not flushing toxins appropriately. ^[1][2] Treatment for low urine output includes: ^[1]

• Diuretics: These medications play a crucial role in stimulating the kidneys to produce more urine, aiding in the elimination of toxins and preventing fluid retention.
• Intravenous Fluid: Careful management of intravenous fluids is essential to avoid overloading the kidneys.

Blood Sugar (Glucose) Management

Immature foals may have difficulty maintaining normal blood sugar levels because of the following physiological limitations:

• Glucose Production: They have limited function of the enzyme that produces glucose, the sugar that is the main source of energy for the body. ^[1][4]
• Glycogen Stores: They also have low glycogen stores, which is the storage form of glucose in the horse’s body. ^[1]

The combined effect is that immature foals are predisposed to hypoglycemia, or low blood sugar. Treatment of low blood sugar involves intravenous glucose administration and regular monitoring of blood sugar levels.

Gastrointestinal Management

In premature and dysmature foals, the gastrointestinal system is often underdeveloped and cannot digest and absorb food properly. Development of the gastrointestinal system relies on functional blood pressure and adequate blood oxygen levels to support the function of the intestinal lining.

Symptoms of gastrointestinal compromise in foals include: ^[1]

• Abdominal distention
• Reduced production of feces
• Reflux of food material from the stomach
• Colic

Immature foals require frequent, small amounts of milk until their gastrointestinal system can handle larger meals. ^[1] In some cases, intravenous nutrition is needed to support the foal’s growth and prevent weight loss. ^[1]

Foals with gastrointestinal compromise are predisposed to developing diarrhea, which can further complicate a treatment plan and the foal’s prognosis. ^[1]

Temperature Management

Immature foals often have difficulty maintaining a stable body temperature and need help to support thermoregulation.

These newborn foals are particularly susceptible to heat loss due to their relatively large surface area compared to their body mass. As blood circulates through their extremities and skin, it is rapidly cooled, leading to a decrease in body temperature for the foal.

If left unmanaged, these foals can rapidly develop hypothermia, a condition characterized by critically low body temperature. ^[1][2][5] Treatment for low body temperature includes: ^[1][5]

• Moving the foal to an environment free from drafts
• Administering warm intravenous fluids
• Using heat lamps or moving the foal into a heated building

Immune System Management

Many immature foals are too weak to stand at birth, which prevents them from drinking colostrum, the mother’s first milk. Colostrum contains high levels of antibodies, which are important for establishing the foal’s immune system.

Foals must drink enough colostrum within 24 – 48 hours of birth to build their immune defenses. Missing this critical window is called failure of passive transfer and puts the foal at an extreme risk of life-threatening infections.

Foals that are unable to stand require nasogastric intubation of colostrum within 24 – 48 hours. If intubation does not occur, or the foal’s antibody levels are still inadequate after colostrum administration, the foal requires a plasma transfusion of immunoglobulins (antibodies). ^[1]

Some veterinarians put high-risk foals, such as immature foals, on antibiotics to further reduce their risk of infection. ^[1]

Musculoskeletal Management

Immature foals often have poor development of the bones in their hocks and knees. The bones in these joints are not completely ossified, or hardened, which puts them at risk of collapse when the foal stands on them. Foals with immature bones require exercise restriction to prevent damage to the bone structure. ^[1]

Many immature foals also have lax tendons and poor muscle tone, resulting in abnormal joint positions when standing. Left untreated, laxity can result in permanent angular limb deformities.

Management strategies for lax joints include: ^[1][6]

• Controlled exercise
• Splinting the limbs
• Shoeing or trimming changes to adjust the angle of the limb

Prognosis

The prognosis for premature and dysmature foals depends on the severity of their condition, the effectiveness of treatment, and whether they develop any infections during their treatment period. ^[3]

With appropriate treatment beginning within the first 24 hours after birth, the survival rate is 80 – 85% for immature foals who do not contract life-threatening bacterial infections. ^[3][7]

In the long term, most immature foals are smaller than their peers for the first 12 – 18 months of life, but often catch up over time. ^[1] Some studies show that immature foals have a comparatively smaller stature for their entire lifetime. ^[8]

Bone Issues

Long-term skeletal problems due to incomplete ossification or joint laxity can occur if not managed appropriately early in life. ^[7]

In one study of foals with incomplete ossification at birth, only 5 of the 20 evaluated foals were sound enough for their intended athletic performance. ^[6]

Athletic Performance

Studies on the performance of sick foals show that affected foals are less likely to enter a performance career, however those that do enter a performance career have similar results to their peers. ^[1][9]

For immature foals specifically, they have lower placings and earnings compared to their peers, even once they enter a performance career. ^[9]

Conformation

A recent study analyzed anatomical measurements of mature horses who were born immature or normal. This study found that immature foals were more likely to have a longer body frame with shorter lower limbs than horses born with normal maturity. ^[8]

The authors suggest that the long-backed conformation may contribute to reduced athletic performance due to an increased risk of lumbar pain. ^[8]

Prolonged Gestation

Foals that are born after their expected due date are described as having prolonged gestation. Many cases of prolonged gestation result in the birth of a normal, healthy foal. ^[3][10]

Causes of prolonged gestation without evidence of dysmaturity include: ^[3][10]

• Individual variation in gestational length between mares
• Inaccurate breeding date record, particularly in pasture breeding scenarios
• Delayed embryonic development during the first two months of gestation

No treatment is needed so long as the pregnancy appears to be advancing properly, with normal mammary development. ^[3][10] Most foals with prolonged gestation periods are a normal size at birth and do not carry an increased risk of dystocia in the mare. ^[3][10]

Postmaturity

Postmature foals are foals that are retained in the uterus for too long, causing them to continue growing and maturing beyond what a normal newborn foal looks like.

This extended gestational period can lead to physical signs of over-maturity, birthing complications, and difficulties in standing and nursing.

Causes

The most common cause of postmaturity is fescue toxicosis in the mare. ^[1][3] This condition is caused by consumption of endophyte-infected tall fescue grass.

Fescue toxicosis may block the production of hormones within the fetus that normally trigger delivery, resulting in prolonged gestation. ^[3][5]

Symptoms

Clinical signs of postmature foals include: ^[1][5]

• Large body frame but a normal birth weight
• Poor muscle development
• Long tail and a long hair coat
• Erupted front teeth
• Contracted tendons

These foals often have a lanky or emaciated appearance, as they do not receive adequate nutrition in utero to sustain their growth. ^[1]

Treatment

The primary concern with postmature foals is that their large size greatly increases the risk of dystocia (difficulty foaling) in the mare. Without immediate intervention, these foals may experience inadequate oxygen levels during delivery, potentially resulting in dummy foal syndrome.

Mares with fescue toxicosis also commonly experience premature placental separation, also known as a “red bag delivery.” ^[1] In this scenario, the placenta separates from the uterus before the foal is delivered, cutting off the foal’s oxygen supply. ^[1]

Rapid intervention is necessary to deliver the foal and prevent suffocation. ^[1] Red bag deliveries also have a high risk of dummy foal syndrome.

Prognosis

Foals that survive dystocia and do not develop dummy foal syndrome typically have a good prognosis. ^[1]

However, fescue toxicosis often results in agalactia (lack of milk production) in the mare. This can increase the foal’s risk of failure of passive transfer, resulting in impaired immune function. ^[1]

Careful veterinary evaluation of postmature foals is required to ensure long-term health.

Frequently Asked Questions

Here are some frequently asked questions about foal maturity and gestation periods in horses:

Summary

• Immature foals have a silky hair coat, floppy ears, small size, and poor muscle development
• Postmature foals have a large size, long hair coat, and erupted front teeth
• Premature, dysmature, and postmature foals require additional evaluation and treatment after birth
• Prolonged gestation does not usually have adverse effects so long as the pregnancy is proceeding normally