Dr Mina C.G. Davies Morel, University of Wales, Aberystwyth

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Incidence of twin pregnancies

Twinning is an increasing problem in stud management, especially in intensively bred horses such as Thoroughbreds, Arabs and Warmbloods. Twin pregnancies result from the fertilisation of two ova. The incidence of twin ovulations, and, therefore, twin pregnancies, varies significantly with breed and age of mare. It has been reported that only 1% of oestrous cycles in pony mares result in a multiple ovulation (MO) whereas in Thoroughbred, Arab and Warmblood horses up to 40% of cycles demonstrate MO. The incidence of MO also increases with age. All these ovulations have the potential to be fertilised and result in pregnancy. Mortality of some of these twin embryos does naturally occur, as will be discussed later, but as most mares are scanned very early in pregnancy (Day14-20 p.c.) the incidence of twin pregnancies is very similar to, and follows the same pattern as, multiple ovulation. As all these pregnancies have the chance to develop further and it is not possible at this stage to accurately identify those that will naturally die, all twin pregnancies must be considered to be of concern. Additionally it is increasingly evident that twinning is a heritable trait.

Why are twin pregnancies a problem in the mare?

The mare is classified as monocotous, her uterus only being able to adequately support a singleton. The placenta for that pregnancy needs to attach to the entire surface of the uterus in order to satisfy the demands of the foetus, especially in the last trimester.  The development of a twin pregnancy results in the surface of the uterus available to each placenta being reduced, so compromising foetal development and ultimately survival. Hence twin pregnancies rarely survive to term, most commonly resulting in abortion in mid to late pregnancy (9-10months). They are in fact the most common cause of non-infectious abortion, accounting for 20-30% of all occurrences. Only 9% of twin pregnancies survive to term: 64.5% of which result in 2 dead foals, 21% in one live foal and 14.5% in 2 live foals.

Twin pregnancies, like multiple ovulations can be classified as unilateral, both in the same horn, or bilateral one in each. 70% of pregnancies are reported to be unilateral and 30% bilateral. With regard to the possible prognosis of a twin pregnancy they can be further classified into: i) equal division of the uterus (50%:50%); ii) minor unequal division of the uterus (40%:60%); iii) or major unequal division of the uterus (20%:80%) (Figure 1).

i) Equal division of the uterus (50%:50%)

If the division of uterine surface area available to the placenta of each twin is equal, then the pregnancy is bilateral and both twins have an equal chance of survival. However, survival is in no way guaranteed and due to placental restriction their birth weights of any surviving twins will be reduced. Such foals often die at or soon after birth and if they do survive rarely achieve their expected mature size or performance or race potential.

ii) Unequal division of the uterus (60%:40%)

If the division of the uterus is unequal, but only a minor inequality, the pregnancy will be bilateral and it is likely that it will develop normally initially. Problems will become evident when placental restriction affects the smaller twin, often in the last trimester. The smaller twin will die and cause the whole pregnancy to abort.

iii) Unequal division of the uterus (20%:80%).

If the division is unequal, with a major inequality, the pregnancy will be unilateral and one twin will predominate over the smaller twin, which is confined to the upper part of the uterine horn. The smaller twin is unable to develop to any extent and, therefore, dies early on. In such a scenario it is possible that the smaller twin will become mummified and so not cause abortion of the whole pregnancy. The larger twin will continue to develop normally. However, if the smaller twin develops a placenta to any extent and survives to the stage when placental size has been fixed (approximately Days100-150) then after its death, though part of the uterine surface will have been vacated, the placenta of the surviving twin will be unable to take advantage of them as full placental size has already been determined. In such a case a singleton will be born but have a smaller birth weight than might be expected. As the mummified foetus may not be noticed in the placental tissue the reason for a low birth weight may not be immediately apparent.

A) Singleton.

B) Equal split (50%:50%).

C) Unequal split (60%:40%).

D) Unequal split (80%:20%).


Figure 1 Placental configurations in the equine singleton and twin pregnancies

  • A) Singleton.
  • B) Equal split (50%:50%).
  • C) Unequal split (60%:40%).
  • D) Unequal split (80%:20%).


What would happen naturally to twin pregnancies in the mare?

Naturally mares that conceive twins would rarely produce a live foal. The continuance of the trait for twinning within the population would, therefore, be limited. This is largely what has occurred in the more native type pony breeds and why their twinning rates are so low. However, modern techniques such as scanning allow us to manage mares that conceive twins, as discussed below, in such a way that they can produce offspring but this results in a perpetuation of the twinning trait within the population. Hence intensively managed mares have the highest twinning rates. If a twin pregnancy in any breed in monitored a significant natural reduction (loss of one embryo) does occur in early pregnancy. In this reduction the position of the pregnancies is important as 85% of unilateral pregnancies reduce naturally at around Day18, but very limited reduction of bilateral pregnancies occurs.

Managing the mare that conceives twins?

As the conception of twins invariably does not result in viable offspring it is advantageous to identify and manage twin pregnancies as early as possible, especially if recovering is planned. The advent of ultrasonic scanning has significantly helped this aim.

There are 4 main management practices used to reduce the incidence of twins: i) monitor ovulation; ii) wait and see; iii) manually reduce; or iv) treat with PGF2a.

i) Monitor ovulation.

Historically, the incidence of twinning was reduced by monitoring ovarian activity using rectal palpation and withholding covering from mares with more than 1 large follicle. The mare would then be covered on the next natural or the next artificially advanced oestrus. This successfully reduced twinning rates within a population but with it conception rates declined and the time interval between parturition and successful covering increased. In order for these drawbacks to be addressed, identification and treatment of actual twin pregnancies rather than potential twin pregnancies is required.

ii) Wait and see.

As up to 85% of unilateral twin pregnancies are reported to reduce by Day 18 one option is to monitor the pregnancy and observe if natural reduction occurs. If not induced abortion at a later stage may be advocated. The advent of scanning now allows such monitoring to occur easily.

iii) Manually reduce

An alternative to natural reduction is to manually reduce. Manual reduction of twins to a single has been reported to be up to 96% successful between days 13 and 16. Manual reduction involves the manual squeezing of the smallest embryo, identified by ultrasound, either between the thumb and forefinger or by using the scanner probe to push the conceptus against the uterine wall and pelvis until the vesicle ruptures. This is best done prior to fixation (Day18) so is normally carried out at initial scanning Days11-12. After fixation the manual reduction of bilateral twins can still be very successful but reduction of unilateral twins runs a higher risk of loosing the whole pregnancy. Other methods of manual reduction at later stage of pregnancy (after Day 40) have been reported such as ultrasound guided allantocentesis and transabdominal foetal cardiac puncture, but have not proved as sucsuccessful as early manual reduction.

Figure 2. The appearance of twins on an early ultrasonic scanning picture. Note the two dark spheres indicating two embryonic vesicles. 

iv) Treat with PGF2a

An alternative to manual reduction of one twin is to artificially induce abortion of the whole pregnancy and recover the mare at the next advanced oestrus. Abortion and subsequent return to oestrus and ovulation can be induced using a single injection of PGF2a, multiple injections may be required later on in pregnancy. Abortion can be induced prior to the next expected oestrus, ie before Day21 of pregnancy. This is often done at the time of first scanning which may be as early as Day11. In which case the mare’s return to pregnancy post abortion may be delayed by only 15 days or so. Alternatively, the pregnancy may be allowed to progress longer in the hope that natural reduction may occur before PGF2a is required. However, if a rapid return to oestrus is required then PGF2a must be administered prior to the development of the endometrial cups at Day40.


The high incidence of twin pregnancies is of considerable concern. The advent of modern techniques such as ultrasonic scanning and hormone therapy allows us to effectively manage mares with twin conceptuses. However, the industry must be aware that in allowing mares that habitually throw twin pregnancies to reproduce, the trait for twinning is being perpetuated within the equine population and with it the need for human intervention.

*If you are interested in helping current research into the neurochemistry of equine stereotypy, veterinarians or horse-owners in the unfortunate position of having to put down either stereotypy or non-stereotypy animals can donate brain material by contacting The University of Wales Aberystwyth on 01970 621690 or by e-mailing sdm@aber.ac.uk


Dr. Mina Davies Morel

{ Dr. Mina Davies Morel is responsible for the Masters in Equine Science at The University of Wales, Aberystwyth. More information about the course and others like it can be found by clicking on the link here: http://www.wirs.aber.ac.uk/brochure/msc_equine_science.shtml }


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