Twin pregnancies in horses are common but carry significant risks for both the mare and the unborn foals. The horse’s reproductive system has developed to carry one fetus to term, and the addition of a second fetus results in competition for nutrients, oxygen and space within the womb.

In most cases, twin pregnancies end in abortion of both fetuses during mid-gestation. If the foals survive, they have an increased risk of health complications and a higher mortality rate compared to singleton foals. Mares that have given birth to twins also have an increased risk of retained placenta and traumatic injury during foaling.

The diagnosis of twin pregnancies is simple and rapid due to widely accessible ultrasound technology in veterinary clinics. In most cases, twinning is managed by terminating one of the embryos or fetuses, or by terminating the pregnancy entirely. Methods for termination include manual embryo crush (“twin pinching”), twin puncture, or craniocervical dislocation.

Early diagnosis is key for a successful twin reduction that allows survival of the remaining singleton foal. Mares should be examined between day 14 and 16 of gestation to identify the number of embryos present, as twin pinching has the best success rate at this stage.

Twinning in Horses

Twinning in horses refers to carrying and delivery of two foals instead of the usual single foal. Unlike other mammals, such as dogs or sheep, horses are not normally capable of carrying multiple fetuses successfully.

In most cases, twin pregnancy results in abortion of one or both fetuses, nullifying the time and expense invested in achieving pregnancy for breeders. Historically, twinning has been a common cause of equine abortions, representing between 20-40% of abortion cases. [1]

More widely accessible ultrasound technology has decreased the incidence of twins in horses, by allowing for rapid and easy diagnosis of twins early in pregnancy. [2] As a result, twin pregnancies now account for only 3% of equine abortions. [3]

The Normal Reproductive Cycle

Mares are seasonally polyestrous, meaning that they undergo multiple heat cycles on a seasonal schedule. Typically, mares cycle when days are getting longer in the spring and summer.

During a normal reproductive cycle, the mare’s ovaries are continuously developing follicles, structures which contain a single egg. At any given time, follicles at various stages of development can be found on a mare’s ovary.

When a follicle is fully matured, ovulation occurs, where the mature follicle ruptures and releases the egg into the oviducts. In most cases, only one follicle ovulates per estrous cycle.

Shortly after ovulation, the egg is receptive to fertilization by sperm, resulting in an embryo. Embryos are found freely floating in the uterus up until Day 16, when they begin “fixation” and start forming a placental attachment to the mare.

Fetal development continues until a minimum of 320 days of gestation, when the foal is considered developed enough to survive outside of the womb. [4]

How Twins Develop

Twins in horses are almost exclusively dizygotic, meaning they develop from two separate eggs and produce non-identical twin foals. [4] Therefore, two eggs must be available for fertilization in order for twins to develop.

In most cases, twins occur when a mare undergoes two ovulations within a single estrous period. [5] These ovulations can occur either simultaneously, with ovulation of two mature follicles at the same time, or with 2-10 days in between ovulations. [5]

In the latter case, sperm introduced at the time of the first ovulation may remain viable in the mare’s uterus until the second ovulation occurs, allowing fertilization of the second egg. [5]

For this reason, some veterinarians believe that stallions that are highly fertile may be more likely to produce twin pregnancies due to increased sperm survival in the reproductive tract. [5]

Identical Twins

Rarely, twins can develop from a single fertilized egg, usually from splitting of the embryo into two viable embryos. These twins are often called “identical twins”, as they share the same DNA due to arising from the same embryo.

Monozygotic twins are particularly rare in horses, likely due to embryos forming a capsule at Day 6 of pregnancy. The capsule likely prevents pinching of the embryo, which can cause splitting into two embryos. [4]

There is some evidence to suggest that embryo transfer pregnancies are more likely to result in monozygotic twins. [4]

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn - Equine Nutrition Consultants | Mad Barn USA

Complications of Twinning

Twinning is a common cause of equine abortion, as the mare’s placenta and uterus is designed to support one fetus. With twins, the two fetuses compete for nutrients and oxygen from the mother, often resulting in death of one or both fetuses in around 85% of cases. [1]

Twin foals that successfully develop have an increased risk of dystocia (difficulty foaling) during birth, which can result in death of either foals or the mare. [1] Successfully born foals are often small and weak and have a higher mortality rate than singleton foals. [6]

Mares delivering twin pregnancies also have a higher risk of retained fetal membranes, which can cause potentially life-threatening complications after birth. [1]

All of these factors make twin pregnancy a health risk for both the twin foals and the dam, as well as a significant economic risk for horse breeders. For these reasons, veterinarians recommend twin reduction or termination of the pregnancy, rather than allowing the pregnancy to continue.

Risk Factors

There are several identified risk factors that increase a mare’s risk of double ovulation, including breed and genetics, as well as past reproductive history.

For example, Thoroughbreds have the highest incidence of double ovulation, occurring in 15-25% of heat cycles. [7] Pony breeds, Appaloosas, Arabians and Quarter Horses have lower rates of double ovulation. [7]

There is also some evidence that certain bloodlines within a particular breed may be more likely to double ovulate. [5]

Open and maiden mares also have an increased risk of twin pregnancy compared to mares who have a foal at foot, likely due to lactation and nursing having a suppressive effect on the reproductive system. [5]

Seasonality may also play a role in twin pregnancy risk, as some reports indicate mares bred during peak breeding season (April-July for the Northern hemisphere) have an increased risk of double ovulation. [8]

Finally, medically-inducing ovulation as part of breeding management is more likely to produce a double ovulation. [4]

Diagnosis

Twin pregnancy can be diagnosed between 10 to 16 days after ovulation by ultrasound. [1]

During this time period, the embryos still move freely within the uterus, and have not yet begun to form a placenta. The embryos appear as black fluid-filled sacs within the uterus on ultrasound located anywhere from the tip of the uterine horns to the cervix. [1]

This period is the ideal time to diagnose twin pregnancy, as it offers the safest options for twin reduction (removal of one twin) or terminating the pregnancy. [1] With further pregnancy development, two separate fetuses and placentas may be identified via ultrasound.

Treatment

There are several methods for managing twin pregnancy in horses, depending on the age of the fetuses when the twins are identified. Most veterinarians recommend ultrasound examination between 14-16 days of gestation to identify twins early and allow for prompt treatment.

Natural Reduction

As horses are not designed to carry twin pregnancies, their reproductive physiology has a mechanism for naturally reducing an embryo and allowing the pregnancy to progress with only one fetus.

Studies show that 85% of twin pregnancies are naturally reduced to a singleton pregnancy after 16 days of gestation. [1] Nutrient deprivation to one of the two embryos is thought to cause death of one fetus, allowing the other to persist and be carried to term. [1]

The natural reduction mechanism appears most effective when the embryos are unequal in size, which allows the larger embryo to dominate nutrient absorption. [4]

Limitations

Natural reduction mechanisms appears to require close proximity of the two embryos so they compete for nutrients, such as when embryos reside in the same uterine horn. [4]

In female horses, the uterine horn refers to the paired structures that extend from the main body of the uterus and provide a site for embryo implantation and development. Twin pregnancies where the embryos are in separate uterine horns rarely reduce naturally.

Natural reduction of twins only appears possible until around 45 days of gestation, at which the likelihood of reduction to a singleton pregnancy drops to around 6%, the normally anticipated pregnancy failure rate at this stage. [1]

For this reason, most practitioners recommend pursuing twin reduction strategies or termination of pregnancy after 45 days of gestation if the “wait and see” approach was unsuccessful.

Manual Crush or “Twin Pinch”

Manual crushing of one of the two embryos prior to Day 16 of gestation has the greatest success rate in maintaining a healthy singleton pregnancy. [1]

During this procedure, the two freely-floating embryos are separated and one embryo is ruptured by applying pressure from the ultrasound probe. [1][4] Typically, the smaller of the two embryos is crushed.

Your veterinarian may follow up with anti-inflammatory treatment and altrenogest to help maintain the remaining singleton pregnancy.

Manual Reduction

In fetuses between 30 and 50 days of gestation, repeated trauma to the fetus transrectally may be sufficient to cause fetal death. In this procedure, pressure is applied to the fetus multiple times over a period of 5 to 10 days with the goal of causing damage to the fetal membranes, terminating the fetus. [1][4]

In some cases, oscillation is used where the fetus is rapidly vibrated until the weight of the fetus causes a whiplash effect that separates it from the umbilical cord. [4] The success rates of these procedures is about 65%. [4]

Transvaginal Ultrasound-Guided Twin Puncture

Transvaginal ultrasound-guided twin puncture is a complicated procedure that can be used between Day 20 and 35 of pregnancy. [1][4]

In this procedure, a specialized needle is guided into the fetal membranes of one fetus, and the fetal fluids are removed. [4] By removing the fetal fluids, the fetus dies and is resorbed.

The success rates of this procedure range between 30-70%, with many mares aborting both fetuses. [4]

Craniocervical Dislocation

In this procedure, the head of one fetus is dislocated from the spine through careful manipulation, resulting in fetal death. Between Day 55 and 90 of gestation, this procedure can be performed via the rectum by experienced practitioners.

Standing laparotomy, where an incision is cut into the mare’s flank, can also be performed between Day 58 and 110. Both methods have around a 60% success rate of producing a healthy, single foal. [9]

Pregnancies where this procedure is performed require ongoing monitoring to ensure the decapitated fetus is terminated, as there have been reports of fetal survival up to 7 months after the procedure. [1]

Transcutaneous Ultrasound-Guided Twin Reduction

In this procedure, one of the twins receives an injection of potassium chloride into the heart, resulting in death of the fetus. [1] The success rate of this procedure ranges from 38-56%.

The remaining foal may be born small and weak, as its twin will have developed enough placental tissue prior to death to prevent the surviving foal’s placenta from developing appropriately. [1] This procedure is typically carried out between 115 and 150 days of gestation. [1]

Pregnancy Termination

The ability to terminate a pregnancy and rebreed the mare within the same breeding season exists up until Day 35 of gestation. [1] After Day 35, the placenta has developed enough to begin producing hormones that suppress normal estrous cycling, regardless of whether the pregnancy is viable. [1][4]

Therefore, pregnancy termination through a prostaglandin injection is typically carried out prior to Day 35.

In cases where twin pregnancy is missed, or twin reduction techniques are unsuccessful, most veterinarians support termination of a twin pregnancy rather than allowing the pregnancy to continue. [1]

Pregnancy termination should be carried out as soon as possible after 100 days of gestation, as natural reduction of the twins is highly unlikely after this stage of development. [1]

Prevention

There are no known methods for preventing twin pregnancy from developing, as double ovulation can occur at any time.

However, ultrasound examination of a recently bred mare between Day 14 and 16 of pregnancy allows for early diagnosis of twinning so your veterinarian can attempt twin reduction techniques. Timely intervention is critical to result in a viable singleton pregnancy.

Horse owners interested in breeding their mare should discuss breeding management strategies with their equine veterinarian. Learn more about how to prepare your mare for pregnancy in this guide: Preparing Broodmares for Breeding.

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. Crabtree. J. R., Management of Twins in Horses. In Practice. 2018.
  2. Ginther. O. J., How Ultrasound Technologies Have Expanded and Revolutionized Research in Reproduction in Large Animals. Theriogenology. 2014. View Summary
  3. Ricketts. S. W. et al., A Review of the Causes of Abortion in UK Mares and Means of Diagnosis Used in an Equine Studfarm Practice in Newmarket.. Pferdeheilkunde Equine Medicine. 2001.
  4. McKinnon. A. O. et al., Equine Reproduction. Wiley-Blackwell, 2010.
  5. Ginther. O. J., Twinning in Mares: A Review of Recent Studies. Journal of Equine Veterinary Science. 1982.
  6. Jeffcott. L. B. and Whitwell. K. E., Twinning as a Cause of Foetal and Neonatal Loss in the Thoroughbred Mare. J Comp Pathol. 1973. View Summary
  7. Chavatte. P., Twinning in the Mare. Equine Vet Educ. 1997.
  8. Merkt. H., More Twin Pregnancies as Season Progresses. Journal of Equine Science. 1999.
  9. Wolfsdorf. K., Management of Twins in the Mare. Equine Veterinary Education. 2012. doi: 10.1111/j.2042-3292.2011.00271.x.