STUD NOTES

ABOUT TWINS IN THOROUGHBRED BREEDING

Twin conceptions used to be considered the 'bogey man' of reproductive problems. Certainly, twin 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 are now handled 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 one fetus to develop. A fetus 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 drugs such as Chorulon and Ovuplant. These hormonal drugs encourage the ovulation of a 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 proficiency of veterinarians using them, it has now become evident as to 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. Draught and thoroughbred mares have a much higher number of multiple ovulations, than say, at the other end of the scale with 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.

EXAMPLE OF A SYNCHRONOUS OVULATION		EXAMPLE OF A ASYNCHRONOUS OVULATION
Two follicles in the same ovary; both at about the same stage of development. The mare is subjected to an Ovuplant to ensure ovulation within 36 hours from administration. The mare was served 12 hours after the Ovuplant.	Same mare and both follicles have ovulated. The outline of 2 corpus luteum can be seen. This photo was taken 48 hours after the Ovuplant was administered, it is very likely this young fertile mare will conceive twins.	An asynchronous. There is still a follicle on the left of the picture, but corpus luteum beside it.

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' either side of 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 in the same horn (Unilateral fixation) or, at separate horns (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 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 fetus 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 fetus, generally resulting in a well developed, live foal.

(ii) The most common reason for twin abortion is when one fetus out competes the other. The smaller fetus will die in utero, in which case both fetus will be probably be aborted.

When two live foals are born, one will often be smaller and less developed than the other, but by and large resulting in the death of both foals either by natural causes or the basis of a decision made on humane grounds.

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 undersized. 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 is exposed to further reproductive problems. There are often retained foetal membranes, she may not cycle, and she has probably suffered uterine trauma either through internal damage or bacterial invasion.

I have a story where Rick and I were attending a twin foaling and the vet of the time, Dr Des James also happened there. The first foal delivered was about the size of a large cat, and we basically cast it off to one side without further ado: there was a much larger foal following. This larger foal was still undersize, yet nevertheless worth a shot: but barely had a heartbeat. We were unable get this foal going, despite having all necessary equipment at hand and persisting for some time. Next thing we hear a whinny behind Rick, and there was this tiny thing standing (of sorts). The larger foal had the better body maturity, the smaller foal had better organ maturity. This small foal was subsequently euthanaised as survival was just not going to happen. It is the most memorable twin set we've had.

We have not had a set for many years - the above photo being the most recent taken in 2005, and it had been a few years since the prior set. When we first started out we'd have a couple of sets every year. The scan changed that.