Equis ISSN 2398-2977


Synonym(s): Twin pregnancy, multiple pregnancy

Contributor(s): Madeleine L H Campbell, William Ley, Graham Munroe, Vetstream Ltd, Elaine Watson


  • Cause: usually as a result of at least 2 ovulations and ova (dizygotic).
  • More common in Thoroughbreds and large mares.
  • Monozygotic twins have been reported in mares but are extremely rare.
  • Accounts for 20-30% of abortions in Thoroughbred mares. The most common cause of non-infectious abortion.
  • Reported incidence: 0.5-14% or higher of conceptions (higher incidence has been reported using earlier diagnosis with ultrasound).
  • Signs: may be none, precocious lactation, abortion   Abortion: overview  , premature birth, dysmature foal(s)   Reproduction: prematurity / dysmaturity  ; early death of one fetus common (natural embryo reduction)   →   normal singleton fetus.
  • Twin pregnancies often terminate in abortion during the last third of pregnancy or result in the birth of dysmature foals.
  • Diagnosis: ultrasound 14-23 days; rectal palpation   Urogenital: rectal palpation  .
  • Treatment: managed by either the elimination of one conceptus by manual rupture, fetal injection of KCL or procaine penicillin, ultrasound-guided aspiration or craniocervical dislocation, or by the elimination of both conceptuses by exogenous prostaglandin treatment.
  • See also Reproduction: management - female   Reproduction: management - female  .



  • Multiple ovulations.
  • Few cases of identical equine twins have been reported.

Predisposing factors

  • Breed: increased incidence in Thoroughbreds   Thoroughbred  and other larger breeds.
  • Barren mares.
  • Stage of season - increased twin ovualation in late spring/early summer.
  • Rare in native breeds.


  • Multiple ovulation in previous seasons.
  • Individual mare predisposition.


  • Equine twins are usually due to fertilization of twin ovulation and 2 ova.
  • High levels of natural embryo reduction occur before day 40, especially in unilateral twins.
  • The mare's uterus is not designed to carry twins.
  • Most advanced twin pregnancies result in abortion or delivery of full-term dead foals due to the limited ability of the mares uterus to nourish more than 1 foal.
  • The dead fetus may or may not be mummified.
  • Only c 11% of mares carrying twins will successfully produce viable twins at full-term.
  • Multiple ovulation is common in horses.
  • Some mares repeatedly ovulate multiple times in one estrus cycle.
  • Twin pregnancy is usually dizygotic and may follow synchronous (<24 h apart) or asynchronous (>24 h apart) double ovulation.
  • Conceptuses can be detected in the uterus by ultrasound as early as 12-14 days as a circular dark area   Ultrasonography: reproductive tract - female  .
  • Conceptuses are mobile until 16 days when they fix - usually at uterine horn-body junction.
  • Twin conceptuses act as singletons until fixation - the incidence of loss is as for singletons.
  • Embryo reduction minimal until fixation either unilaterally or bilaterally in uterine horn.
  • Embryo reduction occurs mostly from 17-40 days.
  • Incidence of reduction increased where vesicles are fixed unilaterally and when of unequal diameter.
  • "Natural reduction" is much more likely to occur if the embryos are fixed in the base of one horn (84-89% reduction) than if one embryo is fixed in each horn (4% reduction).
  • In 59% of cases reduction is completed by day 20. The sooner reduction is completed the faster the eliminated vesicle disappears.
  • The longer the twin lasts the less chance there is that it will be naturally reduced.
  • Reports of triplet pregnancy are rare and may have been associated with exogenous hormone usage or stallions whose semen exhibits unusual longevity.
  • Most (65%) of advanced twin pregnancies result in abortion or delivery of full-term dead foals.
  • This is related to the type of placenta in the equine (diffuse).
  • The inability of the uterus to support 2 fetuses to term is due to insufficient placental support area with abortion most common after 7 months gestation.
  • If carried to term the combined birth weight rarely exceeds the normal birth weight of a singleton.
  • 3 distributions of twins in uterus:
    • One twin occupies the uterine body and one horn and possesses approximately 70% of functional surface area.
    • The two chorions are in contact (usually a degree of invagination of the smaller into the larger) and therefore devoid of villi in this contact area. This is the most common configuration and often results in abortion or stillbirth of one or both twins in late gestation.
    • Villous surface area is equally divided between the twins each of which occupies one horn and half of the body. Twins may be aborted or born alive, both weak and undersized.
    • Large disparity in villous surface area with the chorion of one twin almost totally excluding the other, the latter dies early in gestation and the other twin born alive ?normal.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Sper R B et al (2012) Successful reduction of a monozygotic equine twin pregnancy via transabdominal ultrasound-guided cardiac puncture. Equine Vet Educ 24 (2), 55-59 VetMedResource.
  • Wolfsdorf K (2012) Management of twins in the mare. Equine Vet Educ 24 (2), 60-61 VetMedResource.
  • Hodder A D J, Liu I K M & Ball B A (2008) Current methods for the diagnosis and management of twin pregnancy in the mare. Equine Vet Educ 20 (9), 493-502 VetMedResource.
  • Wolfsdorf K E (2006) Management of post-fixation twins. Vet Clin Equine 2, 713-725 PubMed.
  • Macpherson M L & Reimer J M (2000) Twin reduction in the mare - current options. Anim Reprod Sci (60-61), 233-244 (Review) PubMed.
  • Meadows S J, Binns M M, Newcombe J R et al (1995) Identical triplets in a thoroughbred mare. Equine Vet J 27 (5), 394-397 PubMed.
  • Bracher V et al (1993) Transvaginal ultrasound-guided twin reduction in the mare. Vet Rec 133 (19), 478-479 PubMed.
  • Ginther O J (1989) The nature of embryo reduction in mares with twin conceptuses: deprivation hypothesis. Am J Vet Res 50, 45-53 PubMed.
  • Ginther O J (1989) Twin embryos in mares 1 - from ovulation to fixation. Equine Vet J 21 (3),166-70 PubMed  
  • Ginther O J (1989) Twin embryos in mares 2 - post fixation embryo reduction. Equine Vet J 21 (3), 171-174 (two papers explaining early embryonic activity and the mechanisms of natural embryo reduction) PubMed.
  • Pascoe D R & Stover S M (1989) Surgical removal of one conceptus from fifteen mares with twin conceptuses. Vet Surg 18, 141-145 PubMed.
  • Pascoe R R, Pascoe D R & Wilson M C (1987) Influence of follicular status on twinning rate in mares. J Reprod Int Suppl 35, 183-189 PubMed.
  • Whitwell K E (1984) Triplet pregnancy in two thoroughbred mares. Equine Vet J 16 (4), 393-396 PubMed.
  • Simpson D J, Greenwood R E, Ricketts S W et al (1982) Use of ultrasound echography for early diagnosis of single and twin pregnancy in the mare. J Reprod & Fertil Suppl 32, 432-439 PubMed.

Other sources of information

  • Wolfsdorf K E, Rodgerson D & Holder R (2005) How to Manually Reduce Twins Between 60-120 Days Using Cranio-Cervical Dislocation. In: Proc AAEP Congress. pp 264-267.
  • McKinnon A O & Rantanen N (1998) Twins. In: Equine Diag Ultras. Williams & Wilkins. Baltimore (MD). pp 141-156.
  • Rantanen N W & Kincaid B (1988) Ultrasound-Guided Foetal Cardiac Puncture: A Method of Twin Reduction in the Mare. In: Proc AAEP Congress. pp 173-179.