Two’s A Crowd - Twinning in Mares
by Dr. Allan Gunn, Dr. John Chopin and Dr. Jim Rodger.

There are a number of factors that predispose a mare to conceive twins, however the news that a mare has a twin pregnancy is not a case for celebrating the fact there are two foals for the ‘price’ of one. A twin pregnancy is never desirable in a mare. Unlike other species, the development of twin foals usually results in abortion or the still-birth of either one or both of the foals, as well as causing adverse effects on the mare and heartbreak for the people involved.

The placenta of the mare and its difference from that of other species is the main reason as to why the large majority of twin pregnancies in horses end badly. In all other species twins can be carried successfully because there is space on the uterine wall for the attachment of more than one placenta, however in the horse, the foetus requires the whole uterine wall. In order for the foal to develop properly, the placenta, which acts as a transfer layer for oxygen and nutrition between the womb (uterus) of the mare and the foal, must cover all of the uterus, not just part of it. If there are two foals developing, the placenta of each foal simply cannot cover enough womb for both of them to receive sufficient oxygen and nutrition to develop properly. The mare will often abort during the final third of her pregnancy when the uterus is no longer able to support both foetuses due to the inevitable reduction of the available placental area. Even if the foals do go to term, they will often be still-born, and if either or both are born alive, they are normally small and under-developed and usually need intensive care to survive. It is rare for both twins to survive - despite occasional media reports to the contrary – and twinning remains an important cause of mid-to-late term foal abortion and still-birth.

Mares that carry twins to late pregnancy are more likely to have retained foetal membranes (retained placenta), and have difficulty conceiving and maintaining the next pregnancy.

Why Twins Occur
During ovulation an egg is released from the mare’s ovaries, which is then fertilised by sperm from the stallion and travels into the womb to develop into a foal. The developing embryo enters the uterus approximately six to seven days after ovulation. It moves around the whole uterus so that the reproductive system is aware of its presence, allowing it to develop, and preventing further cyclicity (known as Maternal Recognition of Pregnancy). At about 16-17 days after ovulation, this phenomenon ends, and the developing embryo (vesicle) becomes ‘fixed’ in one part of the uterus, typically at the base of either uterine horn. Twins occur when more than one egg is released, and both, or more, are fertilised. This is even more likely if the eggs are released at around the same time (synchronous ovulation), and one being from each ovary. Most mares only release one egg (one ovulation) each time they are in season, but there are a number of factors that make it more likely for more than one ovulation to happen, possibly resulting in a twin pregnancy.

Larger breeds, such as Thoroughbreds, draft horses and Warmbloods, are more likely to conceive twins than ponies. Typically, Quarterhorses and Standardbreds fall somewhere in between the two. Some particular individuals, and even some families appear to be more likely to develop a twin pregnancy, while the risk of twinning is also increased in maiden, barren  and older mares. There is debatable evidence to suggest that more ‘fertile’ stallions may have a greater chance of causing twins in mares.

Twins aborted at 270 days (9 months) after the mare had shown signs of premature udder development.
Photograph by Nicky Holdstock.

As the breeding season progresses, mares are more likely to develop twins, especially when hormones and drugs such as Prostaglandin (‘PG’) and hCG/Chorulon®, deslorelin/ovuplant® are used to shorten cycles or assist ovulation prior to the mare being served. The ovulating drugs tend to induce more synchronous ovulations from follicles that may not otherwise ovulate ‘naturally’. The inherent peak of fertility in mid breeding season, reflected in a slight increase in twin/multiple ovulations, is associated with a combination of environment and the use of drugs. Warmth is probably one of the least important factors in a mare cycling, with the single most important factor being day length.

Detecting Twins
As neither owners nor vets can prevent a mare from having more than one ovulation in her cycle, the emphasis is instead on the early detection of twins, which relies on a veterinary examination, preferably at an early (less than 16 days after ovulation/mating, but usually more than 12 days after mating) stage of the pregnancy. The recognised method to deal with a twin pregnancy is an ultrasound scan of the uterus via the rectum at around 16 days after breeding. Detection and ablation (squashing) of the smaller of the two early developing embryos (vesicles) can then be carried out. Most vesicles are about 14 mm in diameter at approximately 14 days post ovulation and tend to grow at approximately 3 mm per day until the capsule starts to weaken at about 20 days post ovulation.  So ablation tends to occur at the size between approximately a 5 and 10c piece as seen on the ultrasound screen. Any earlier than this may mean that the vesicle/s is/are too small to be seen, and any later than this may result in twin early pregnancies fixing next to each other in the uterus.  Up to this stage they  move (motile) freely within the uterus. Once they are fixed, they can’t be separated, and squashing one can result in damage to both. Some veterinarians will administer sedatives, and/or an anti-inflammatory agent and/or a hormone for the twin squashing procedure, although this has been shown to be unnecessary in most cases.

It is known that ultrasound detection of twins is not perfect. Equally sized twins can overlie each other and appear as one, and a small twin may be too small to visualise on the ultrasound screen. For an ultrasound examination (scan), the mare needs to be adequately restrained, usually in a crush at either the veterinary clinic or on the property. The light needs to be dim without any reflections, so that the screen can be seen clearly; for this reason a mare crush is often placed in a barn or shed. The vet is usually experienced in equine reproduction and ideally has followed the mare’s cycle before and after breeding. Veterinary input prior to mating is important for a number of reasons. These include monitoring the number of developing follicles on each ovary and recording any cysts - which can look like a pregnancy - that may be in the womb. It is also more likely to increase the chance of the mare becoming pregnant by providing timely and necessary treatments around mating.

SCANS LEFT: Approx 20 and 25 day conceptuses adjacent to each other. Note the similarities
to the cyst and vesicle in the next  ultrasonogram.  CENTRE: 14 day embryo below an endometrial cyst.
RIGHT: Two synchronously ovulated vesicles next to each other at approx 14 days post service.

Managing Twin Pregnancies
Reports indicate that if twins are dealt with prior to fixation at 16 days, then at least 60% of mares will give birth to a live single foal. In the authors’ opinion, it is more likely to be similar to a normal pregnancy diagnosed at 16 days, where more than 70% of mares give birth to a live foal at the end of pregnancy. There is a possibility, with many stating the chances being up to 80%, that twins may self reduce, via a deprivation theory, to a single pregnancy. If they ‘self reduce’ early in pregnancy, say before 80 days, then there will be very little sign, if any of the twin having been present. The foetal fluids appear to be absorbed with the death of the vesicle/foetus, with no ill effects.
As the foetus ages, up to say 250 or so days, mummification is likely to occur. Depending on the case and time of death, the mummy may have very little effect, or it may have taken up enough placenta/womb to induce a relatively decreased size of surviving twin.

After about 250 days, abortion or stillbirth is the most likely scenario. This is more likely to occur if they are from synchronous (at the same time) ovulations (hence very little difference is size of vesicles), and if they are not next to each other in the uterus. They ‘fix’ at the base of each horn, and are not able to exert a negative (deprivation) effect on each other early in pregnancy.

If twins have not ‘self-reduced’ to one pregnancy by 30 days, it may be the best option to abort both pregnancies and start again. It is almost impossible to select only one at this stage of pregnancy, however if one was removed and the other did live for a few days only, the chances of successfully rebreeding the mare would diminish due to the production of a hormone that causes haemorrhage of follicles without ovulation.

Reducing twins to a single pregnancy after 45 days is possible, but with less than a 50% chance of success. These options are usually limited to veterinarians with specific expertise or specialist qualifications in the area of equine reproduction. The procedures are more dramatic, with most requiring some sort of surgical intervention.

Despite all this time, effort, cost and following the previously outlined procedures, it is still possible for twins to occur.  Although it is a far less likely occurrence if veterinary intervention is early in the pregnancy, and it is better to be prepared for any outcome to avoid further expense and emotional turmoil.

ABOUT THE AUTHORS (TWINNING)

Dr. Allan Gunn BSc.Agric; BVM&S; MACVSc (horse medicine; reproduction); Diplomate ACT; MRCVS. Allan, from Barn Veterinary Services, Muswellbrook, NSW, has  practiced veterinary medicine in the UK, Zimbabwe, China and Australia, with the last 10 years as an equine stud veterinarian. He went on to achieve MACVSc qualifications in horse medicine and reproduction and became a Diplomate of the American College of Theriogenologists (a specialist qualification in reproduction) in 2008.

Dr. John ChopinBVSc; PhD; FACVSc (equine reproduction); Registered specialist in equine reproduction. John completed a residency in equine medicine and surgery then a PhD in equine frozen semen and the endocrinology of ovulation in the mare. A theriogenology residency followed, this included a teaching sabbatical at The University of California Davis. From here John completed his Fellowship and joined Coolmore Australia in the Hunter Valley, NSW as an equine reproductive specialist.

Dr. Jim Rodger BVMS; MACVSc; MRCVS; FACVSc (equine reproduction). A Fellow of the Aust. College of Veterinary Surgeons, he is a registered specialist in Animal Reproduction (Equine), has  a Post Graduate in Equine Medicine and has  his own practice  - Rodger and Associates in Jerry’s Plains, NSW.

Ackowledgment: Dr. Victoria Brookes for literary input.
For more information on the Equine Veterinarians Australia (EVA) go to: www.eva.org.au