Twin conceptions used to be considered the big "bogey man" of reproductive problems. Certainly, as always has been the case, twins conceptions that evade detection can produce adverse outcomes - serious results in some cases.

These days multiple conceptions are looked at in a very different light. Indeed, the mare that is likely to produce, or does produce a multiple conception, is viewed by some as being a highly fertile mare! This attitude has evolved almost solely as a result of the ultrasound scan. It has enabled vets to become more efficient in detecting the possibility of, and accurately diagnosing twins. The ultrasound has also enabled the vet to be considerably more proficient at eliminating a twin pregnancy.  Married with good management and record keeping by the stud master twin conceptions arenow managed in a way that due foaling dates are not compromised, abortive measures are rarely used, and the successful development of a single embryo to birth is achieved in nearly all cases.

The mare is designed to only allow for one foetus to develop. A foetus requires the placental membrane to cover the whole of the uterine wall for sufficient growth and development. This is a unique feature among all species. However as any breeder will testify, just because the mare is designed with this characteristic, it does not mean that twins can not and do not occur.


WHY TWIN CONCEPTIONS OCCUR
Twin conceptions in the horse are very, very rarely derived from the division of a single fertilized ovum. Multiple ovulations either in the same ovary, or separate ovaries are almost solely responsible for the incidence of twin conceptions. Such ovulations may be synchronous (within 24 hours apart), or asynchronous (2 - 10 days apart).

Incidence of twins may have also been increased in recent years with the more extensive use of HcG drugs, such as Chorulon and Ovuplant. These hormonal drugs encourage the ovulation of mature follicle within a certain time frame. If there is more than one such follicle, we could be increasing the number of synchronous ovulations, hence the probability of twin conceptions.

The pressure on studs to achieve high fertility rates could have a bearing on the incidence of twin conceptions. We are in a position to do as much as we can to encourage a good uterine condition prior to the mare requiring service. This will also enhance the likelihood of two ova becoming fertilized and develop into viable embryos.

Certainly with the introduction of ultra sound scanning devises and the proficiency of the veterinarians using them, it has become evident just how many mares actually conceive twins that once we may have never known about.

It is interesting to note that there is a correlation between the size of horse breed, and frequency of multiple ovulations. Draft and thoroughbred mares have a much higher number of multiple ovulations, than say, at the other end of the scale pony mares. Mares between the ages of 6 years to 10 years have more multiple ovulations, and individual mares will continually have a greater number of multiple ovulations.





WHAT HAPPENS DURING A TWIN PREGNANCY
In a normal pregnancy, the embryo is mobile from day 9 through to day 16. At day 16 it will "fix", or "settle" at the junction of a uterine horn and uterine body (corpus corneal junction). This is referred to as Embryo Fixation. The same applies if there are two conceptions. Both embryos may fix at the same horn junction, (Unilateral fixation) or, at separate horn junctions (Bilateral fixation).  It is not until this stage that a mare is likely to naturally terminate one of her pregnancies, with a high proportion of an embryo discontinuing development between 25 and 40 days.
Unilateral fixation is more common than bilateral fixation. Unilaterally fixed embryos are more likely to naturally reduce to a single viable embryo by day 36 - 40 than bilaterally fixed embryos. This is thought to be the result of the two sets of membranes coming into contact and competition for nutrients.

Bilateral fixation will result in fewer twin conceptions naturally reducing to a single embryo by day 40. These are capable to surviving longer because it is not until they become more developed that the real competition for uterine/placental space, hence nutrient availability, comes into play.

As pregnancy progresses beyond day 40, there is increasing pressure for the embryos to compete for the available uterine wall space.
One of two scenarios may develop during a twin pregnancy;

1. Symmetrical distribution of the placentas - where the two placentas share equal portions of the uterine wall. This may result in 2 live foals being born alive and at full term, but is quite rare.

2. Asymmetrical distribution of the placenta - where one foetus successfully out contests the other for more of the uterine wall. The result of this scenario is dependant on the age of the pregnancies.
(i) If competition becomes too great early in pregnancy for one twin, then it may mummify. The more developed twin will successfully "wall off" the mummified twin overtaking most of the uterine territory. This will allow almost normal development of the single foetus, generally resulting in a well developed, live foal.
(ii) The most common reason for twin abortion is when one foetus out competes the other. The smaller foetus will die in utero, in which case both foetus will be probably be aborted.

When two live foals are born, one will be smaller and less developed than the other, also often resulting in the death of both foals.


WHY TWINS ARE UNDESIRABLE IN THE THOROUGHBRED
Very rarely does it eventuate that two healthy twin foals are born alive.

Usually, if both are born alive one of the foals will be significantly undersize. Vital organs and bones are unlikely to be fully developed in most live twin foals, hence survival rates are limited. Even a quite well developed live twin foal is likely to be very open to infection, and possibly have slower development.

The actual birth of twins can often be complicated by presentation problems, causing future reproductive problems to the mare.

When abortion has resulted the mare often has subsequent reproductive problems. There are often retained foetal membranes, she may not cycle, and she has probably suffered uterine trauma either through damage or bacterial invasion.


MANAGEMENT OF TWIN CONCEPTIONS - What are the options?
At this farm we are not at all concerned about breeding a mare that is likely to have a multiple ovulation. We instead rely on our management to overcome the events that may unfold as a result. There are two extremely important dates we keep in mind when dealing with twins. Sixteen days post ovulation, and thirty six days post ovulation. Their importance will become evident further in the text.

The very first thing we do is map out any cysts in the mares uterus. Cysts experience very little, if any change during the breeding season. Also they do not move around in the uterus. Thus, if we confirm cysts, their shapes and position prior to breeding we are in good stead for accurate pregnancy diagnosis on our very first test. The greatest relevance of cysts to multiple conception diagnosis is that some can look awfully like a pregnancy, or there are so many of them, it makes accurate pregnancy diagnosis very difficult.

We will always breed any mare that is ready for breeding regardless of the fact she is likely to have a multiple ovulation. We are delighted to have multiple ovulations, especially in the mare of questionable reproductive capability. To us it means we have a greater chance of achieving the first objective - to get the mare in foal.

All ovulations are noted. Any other mature looking follicles are also recorded, and ovulation confirmed so that we are ready for a possible asynchronous ovulation that may require two pregnancy tests 4 or 5 days apart to confirm there are not twins. This generally means that we are prepared for the possibility of twins in the mare that has a multiple ovulation.

Despite all efforts, that twin conceptions do eventuate from a mare that appeared to have only a single ovulation. Therefore whenever we pregnancy test a mare, whether she has had a known multiple ovulation or not, we assume there is a possibility of multiple conceptions occurring.

The timing of the initial pregnancy test is important. We like to diagnose and have dealt with twins before they undergo embryo fixation.
The best practice is to have a fairly accurate idea of when ovulation occurred - it is more relevant that the service date in this regard. Assuming diagnosis of twins has occurred before fixation, crushing one of the embryos is a relatively simple procedure. At day 14 the embryos are mobile, and of a size that are easy to diagnose and manipulate. This is an extremely advantageous to the vet when twins occur as we have all options available to us in our subsequent management.

Upon initial confirmation of twins there may be one of three scenarios;
1. The twins may be well apart,
2. They may be close together, but not touching,
or 3. They may be adjacent, and touching.

Scenario one is a simple routine matter of crushing one twin, allowing selection of which is to go. Scenario two is also relatively simple, but one of the embryos needs to be moved away before crushing takes place. Scenario three is more difficult. Diagnosis can be more difficult, as twins may be so close, that only an impression of their presence may be detected. These twins need to be separated before crushing can progress. As the twins are mobile, the simplest and most sensible procedure is to put the mare to one side for a period of time. Usually we will continue to complete the rest of the veterinary session. We will re-present the mare to the vet. Quite often on a second or subsequent look, we find that the twins have moved far enough apart on their own accord. If this is not the case, there is always tomorrow for another look. We are still at 15 days only, and the twins are still mobile.

Some degree of manual separation of the twins is often required of the vet. Carefully manoeuvring one of the twins away from the other, up to the tip of a uterine horn, the embryo is subjected to the pressure of the ultrasound transducer, and the pelvis, and then crushed. The careful vet will do this procedure in a sweeping action, always monitoring the whereabouts of the desired remaining embryo. Generally the veterinarians prefer to perform this procedure with the assistance of a rectal relaxant. An uncooperative mare in the crush can make this a difficult task. Tranquilizers may be used to calm the fractious mare.

Occasionally, it may occur that one of the twins is situated in front of the cervix. The vet may opt to crush the twin at this position, simply for the reason that it is safely away from the other, and its position here allows it.

Once crushing has successfully been achieved we routinely give the mare a drug that has an anti-prostaglandin effect. Whether this is of any help or not is open to debate, but it does make us feel better that we have done all possible to protect the remaining embryo. The mare will be re examined 2 days later to check that growth has occurred (remembering that growth is rapid at this stage and the embryo will have nearly doubled in size in this short space of time). If growth has occurred, then there is no reason that the single embryo has any less chance of survival than a normal pregnancy. An unsuccessful crushing will nearly always result in a mare without any viable pregnancy at this examination.

Should twins be discovered after fixation, and they settle together (as a great many do), then our options are much more limited. The following management will also apply to those twins that are inadvertently present at the 27 day or later pregnancy testing.

If crushing occurs with twins in a close proximity, then we are at great risk of damaging the remaining embryo. The fluid released from the crushed embryo may also affect the viability of the remaining embryo. So unless twins are well apart after fixation, then crushing is possibly not the recommended option. Veterinary research suggests that approximately 70% of twins will naturally result in one embryo remaining by day 40 under these circumstances. So the way to go here is probably sit it out, monitoring the pregnancy. This is the point where day 36 comes into play.

Endometrial cups form between day 36 and day 40. If a pregnancy is aborted after this time there is very little chance of the mare coming back into season to be re-bred that year.

If the time of the breeding season allows for re-breeding, and this is the desired course of action, then abortive measures must take place before day 36 (about 30-32 days). If the time of season does not allow for re breeding, continuing to monitor the pregnancies for one to go naturally is an option. This may extend out to as far as day 60 or 70 before abortion is again reconsidered.

There is one further successful procedure that can be used to reduce a twin pregnancy to a single pregnancy. However it is a surgical procedure that requires an expert to perform. A needle of potassium chloride injected directly into the heart of one of the foetuses. The preferred time frame in which to perform this procedure is between 70 and 110 days.
 
All mares are carefully re examined for the possibility of twins at their routine 27 day pregnancy test. This is especially the case with those mares that had a known multiple ovulations and a single embryo diagnosed on the initial test.
 
With the extremely high success rates ( about 95%) of the experienced veterinarian of manipulating twins in early pregnancy  I see no need to sit and wait on a multiple pregnancy that may or may not sort itself out. We will nearly always opt to crush a twin. If record keeping is accurate, scanning is professional, and mares are presented at the right time, there is little reason to have twins at 17 days!. We have had twins in extremely cystic mares that have got out to 25 - 27 days. The reason for this is that diagnosis is virtually impossible until we have discernable heart beats and embryonic girdles. These mares are the heartbreaking group, as they are probably reproductive problems anyway. Generally this group we will sit on, hoping one will go naturally if crushing is not an option.


MISDIAGNOSIS AND VETERINARIAN PERFORMANCE OF MULTIPLE CONCEPTIONS
Misdiagnosis of twins can and does occur. Most misdiagnosis can be avoided with thorough and professional management.

Misdiagnosis does not automatically mean we point the finger at the vet on the end of the scan. I would say 50% of the responsibility of twin management and combating twin misdiagnosis is done by the person accountable for the mares breeding program - the breeder or the stud.  This means we ask the vet to map cysts, we ask for confirmation of how many ovulations have taken place, and we accurately confirm day of ovulation. We have got to present the mares to the vets at the right time for pregnancy testing - especially if we are pro crushing. We must avail the vet to as much accurate information as possible at the time of pregnancy testing. We most certainly should  not put extra costs associated with possible extra scans performed ahead of need for a good veterinary practice when dealing with possible or confirmed multiple pregnancies. We need to provide the vet with a user friendly environment in which all this takes place, and not place them under any undue pressure on their performance. It is not until now the true skill and onus is solely on the vet - the ability to thoroughly check for twins, and to have the ability to crush one with a high degree of success.