Multiple Pregnancies: Double the Trouble

The causes and management of equine multiple pregnancies.

Multiple pregnancies are an increasing problem in breeding management. They are usually multizygotic (originating from several ova, i.e., the result of multiple ovulations, thus giving nonidentical offspring). Monozygotic twins (originating from a single fertilized ovum that splits to produce identical foals) are very rare, with the first case in England being confirmed as that of two foals born in 2002 to a mare owned by Her Majesty the Queen.

Studies suggest that the vast majority of multiple equine pregnancies are multizygotic twins (94.8%), only 5.2% of multiple pregnancies are triplets, and the occurence of quadruplets is extremely rare.

Our university records indicate that up to 30% of estrous cycles in Thoroughbred mares result in multiple ovulations. Arabian and Warmblood mares also have high incidences of multiple ovulations. However, in a population of native ponies in the United Kingdom, the incidence is less than 1%.

Apart from breed there are other factors--including nutrition, age, and time of year--that seem to affect the incidence of multiple ovulations. Mares that are well-fed and in good body condition have the highest incidence of multiple ovulations, especially if they are over 15 years of age. Work we have carried out demonstrates a clear increase in multiple ovulation rates as mares get older (15% of 3- to 5-year-old mares have multiple ovulations versus 35.1% of 18- to 22-year-old mares). Mares also have a higher incidence of multiple ovulations during the middle of the breeding season (June/July/August in the Northern Hemisphere or December/January/February in the Southern Hemisphere).

Since multiple pregnancies in mares are almost always multizygotic, multiple ovulation is normally a good indicator that there is a high risk of multiple pregnancies, as in theory there is an equal chance of all ovulations being fertilized. However, this might not always be the case. For example, we have demonstrated that as a mare gets older, even though multiple ovulation rates increase, there is a decreasing likelihood that these multiple ovulations will translate into multiple pregnancies. The ovas of older mares have a poorer fertilization rate. Despite this, the presence of multiple ovulations is the first indicator of a potential multiple pregnancy.

Why Worry About Twins?

At first glance, twins would seem to be a good thing, as you would get two foals for the price of one. However, the mare's uterus can only adequately support one fetus at a time. Since all these multiple ovulations have the potential to be fertilized and result in pregnancies, multiple ovulations are a big problem in the mare.

The mare's uterus can only adequately support one fetus because a placenta needs to be attached to the entire surface of the uterus in order to satisfy the demands of the fetus (nutrient and oxygen uptake, waste product removal, and carbon dioxide removal), especially in the last trimester. The development of multiple pregnancies reduces the surface of the uterus available to each placenta and compromises fetal development and, ultimately, survival.

Because of this, multiple pregnancies rarely survive to term, with most resulting in abortion during mid- to late pregnancy.

Before the advent of ultrasound scanning and the resulting ability to manually reduce twin pregnancies (discussed later), twinning was the most common cause of noninfectious abortion, accounting for 20-30% of all occurrences. Naturally only 9% of twin pregnancies survive to term: 64.5% of which result in two dead foals, 21% in one live foal, and 14.5% in two live foals.

The prognosis for a twin pregnancy depends on the extent to which the uterus is shared between the two fetuses. Therefore, twin pregnancies can be further classified into: A) an equal division of the uterus (a 50:50 ratio); B) a minor unequal division of the uterus (60:40); or C) a major unequal division of the uterus (80:20; see page 38).

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

Minor unequal division If the division of the uterus is unequal, but there's only a minor inequality, the pregnancy is likely to develop normally, initially. Problems will become evident when placental restriction begins to affect the smaller twin, often in the last trimester. As a result the smaller twin will die and cause the abortion of the whole pregnancy.

Major unequal division If the division is unequal, with a major inequality, one twin will predominate over another. The smaller twin will become confined to the upper part of one uterine horn. Unable to develop to any extent, it dies early in pregnancy. In such a scenario, it is possible that the smaller twin will become mummified and not cause abortion of the whole pregnancy, allowing the larger twin to continue developing normally. However, if the smaller twin develops a placenta to any extent and survives to the stage when placental size has been fixed (approximately Days 100-150), then after its death, the placenta of the surviving twin will be unable to take advantage of normal placental attachments and support. A single foal will be born, but with a smaller birth weight than might have been expected. The mummified fetus might not be noticed in the placental tissue, so the reason for the low birth weight of an apparent singleton might not be immediately obvious.

How Nature Handles Twins

In nature, mares that conceive twins would rarely produce a live foal. Therefore the continuance of the trait for twinning within the population would be limited. This is largely what has occurred in the native pony breeds and why their twinning rates are so low. However, modern techniques such as ultrasonography allow us to manage mares that conceive twins (discussed below) in such a way that they can produce offspring; however, this results in a perpetuation of the twinning trait. Hence, intensively managed mares have the highest twinning rates.

We've found 70% of twins are unilateral. Of these, 80% will naturally reduce to a single with the death of one of the twins.

Natural reduction (death) does occur in some of these multiple pregnancies, especially before Day 20 of pregnancy. Twin pregnancies can be classified as unilateral (both embryos located in the same uterine horn) or bilateral (one embryo located in one horn and one in the other). The classification of the pregnancy determines the likelihood of natural reduction occurring. We've found 70% of twins are unilateral. Of these, 80% will naturally reduce to a single with the death of one of the twins. However, if the embryos are bilateral, then they both have a very good chance of surviving at least initially, thus, natural reduction is rare.

Options for Twin Pregnancies

As the conception of twins is highly unlikely to result in viable offspring, it is advantageous to identify and manage twin pregnancies as early as possible. The advent of ultrasonography has significantly helped breeders to achieve this aim.

Today most mares are scanned very early in pregnancy (Days 12-14); at this stage it is not normally possible to determine whether a twin pregnancy will reduce to a singleton and, therefore, all multiple pregnancies are of concern.

There are four main management practices used to reduce the incidence of twins: 1) monitor ovulation; 2) wait and see; 3) manually reduce; or 4) treat with prostaglandin F2 alpha (PGF2á).

Monitor ovulation Historically, the incidence of twinning was reduced by monitoring ovarian activity using rectal palpation and withholding covering from mares with more than one large ovarian follicle (indicating that multiple ovulations are about to occur). The mare would then be covered on the next natural or the next artificially advanced estrus, provided only one large follicle was evident. This successfully reduced twinning rates within a population, but with it conception rates declined and the time interval between partrition (birth) and successful covering increased. In order to address these drawbacks identification and treatment of actual twin pregnancies, rather than potential twin pregnancies, is required.

Wait and see Another option is to monitor the pregnancy via ultrasound and observe if natural reduction occurs. If natural reduction does not occur, then abortion of the whole pregnancy can be induced, as the chances of manual reduction being successful are much reduced after Day 20 (as discussed in the following section).

Manual reduction of twins to a single has been reported successful in up to 96% of twin pregnancies between Days 13 and 16.

Manually reduce An alternative to natural reduction is to manually reduce. Manual reduction of twins to a single has been reported successful in up to 96% of twin pregnancies between Days 13 and 16. This involves squeezing the smallest embryo, identified on ultrasound, either between the thumb and forefinger or by using the scanner probe to push the conceptus (embryo) against the uterine wall and pelvis until the embryonic vesicle ruptures. This is best done prior to fixation of the embryo to the uterine wall (Days 18-20), so it is normally carried out at initial scanning during Days 11-12. After Day 20 the manual reduction of bilateral twins can still be successful for a short period of time, but reduction of unilateral twins runs a high risk of losing the whole pregnancy. Other methods of manual reduction at later stages of pregnancy (after Day 40) have been reported, such as ultrasound-guided allantocentesis and transabdominal fetal cardiac puncture, but they have not proved as successful as early manual reduction.

Treat with PGF2á An alternative to manual reduction of one twin is to artificially induce abortion of the whole pregnancy and rebreed the mare at the next estrus. Early abortion and subsequent return to estrus and ovulation can be induced using a single injection of PGF2á, although multiple injections might be required later in pregnancy. Abortion can be induced prior to the next expected estrus, i.e., before Day 21 of pregnancy. This is often done at the time of first scanning, which might be as early as Day 11. If that is the case, the mare's return to pregnancy post-abortion might be delayed by only 15 days or so. Alternatively, the pregnancy could be allowed to progress longer in the hope that natural reduction will occur before PGF2á is required. However, if a rapid return to estrus is required, then PGF2á must be administered prior to the development of the endometrial cups at Day 40.

Take-Home Message

The high incidence of twin pregnancies is of considerable concern. Modern techniques such as ultrasound and hormone therapy allow 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 and, with it, the need for human intervention.

About the Author

Mina C.G. Davies Morel, BSc, PhD

Mina Davies Morel, PhD, is head of the equine group at the Institute of Biological, Environmental and Rural Sciences at Aberystwyth University in the United Kingdom. She has particular interest in equine reproductive physiology and its application to stud management, and she is the author of a number of scientific papers and text books on the subject. She is a leisure rider and owner of Welsh Cob Section Ds.

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