ORIGIN OF TWINS

Equine twins are reported to be dizygotic and arise from multiple ovulations.¹¹ A set of identical, or monozygotic, twins, has been reported but identification was based on a shared chorioallantois, not on blood or DNA type.¹² In contrast to twins in cattle, freemartinism does not occur in equine heterosexual (colt/filly) twins, although blood cell chimerism has been reported.^6,¹³

Multiple ovulations are defined as occurring within a single follicular phase of the estrous cycle.⁴ They may be synchronous, ovulating within 24 hours of each other, or asynchronous, occurring up to 48 hours apart. When follicles ovulate more than 2 days apart, the mare’s estrous status should be critically examined to determine if each ovulation occurred within the same follicular phase. There appears to be little difference in the twin conception rate at the time of fixation between synchronous and asynchronous ovulations,⁴ although in one trial, 96.5% (55 of 57) of mares with twin embryos examined between days 13 and 16 were determined to have had synchronous double ovulations.⁹

Not surprisingly, the incidence of multiple ovulations is highest in those breeds in which twinning is the most prevalent. Besides breed, multiple ovulations are influenced by reproductive status. One investigator found a 50% increase in double ovulations in barren mares over foaling mares.⁹ Multiple ovulations are reported by some to occur more frequently during the late spring and summer,⁴ although solid support for this conclusion is lacking. There is extensive evidence to suggest that double ovulations occur repeatedly in certain individuals,⁴ but the heritability of multiple ovulations is not known. Increasing age has also been associated with a higher risk of twinning.⁵

EMBRYO FIXATION AND REDUCTION

Fixation of the embryo is postulated to happen when the embryos become so large and uterine tone so great that transuterine migration ceases, despite continued uterine contractions.¹⁵ Fixation occurs approximately the same time (day 16 after ovulation) for twin embryos as for singletons; however, ultrasonographic studies found that twin vesicles tended to fix together in one uterine horn (unicornual fixation) more frequently than they were found separately (bicornual fixation). When the size of the vesicles was considered, 85% of embryos that differed in diameter by 4 mm or more fixed unicornually. Similarly sized embryos also showed a preference for unicornual fixation, but by a lesser margin.⁴

Size difference between twin vesicles are likely caused by asynchronous ovulations; thus, a difference in the age of the vesicles. Growth rates of single and multiple embryos are similar, as are diameters of single embryos and those resulting from multiple, synchronous ovulations. Although asynchronous ovulations are not more likely to produce twin embryos,¹⁶ the incidence of unicornual fixation is increased over vesicles resulting from synchronous ovulations.¹⁷

Embryo reduction refers to the biologic mechanism by which one member of a twin set is eliminated.⁴ Evidence indicates that prefixation embryo reduction is negligible; most reduction occurs after day 16.¹⁸ The pattern of embryo fixation greatly influences whether one embryo of a twin set will be reduced before reaching the fetal stage. Approximately 85% of unicornual twin embryos undergo postfixation reduction, more than half before day 20. The day of reduction appears to be later for unicornual vesicles of similar size.

The “deprivation hypothesis” explains the higher incidence of embryo reduction in unicornually fixed twin sets.¹⁹ According to this theory, embryo reduction, as well as the time of reduction, depends on the amount of contact between the endometrium and the vascularized wall of each embryo. Unicornual twins positioned so that one embryo partially or completely prevents contact between the developing vascular wall and embryonic disk of the second embryo underwent rapid reduction that was complete by day 30. Embryo reduction occurs very late in the embryonic stage, or not at all, in bicornual twin sets and unicornually fixed twins that do not compete for endometrial contact. The chance of embryo reduction decreases and the time to complete reduction increases, the longer a vesicle survives after fixation.

BREEDING MANAGEMENT STRATEGIES: DETECTION AND RESOLUTION OF TWIN PREGNANCY

Historically, twin pregnancy was managed by preventing twin conception. Mares with more than one ovulatory-sized follicle (>30 mm) were not bred in the hope that a single dominant follicle would be present during the next estrous cycle. This is not an efficient method of preventing twin pregnancy, especially within the constraints imposed by the operational breeding season, but it was not critically evaluated ⁴ and became a common practice.

A 1982 survey of veterinarians documented the widespread practice of withholding mating to prevent twins.²⁰ Almost all (95%) of the veterinarians surveyed attempted to modify the breeding program in some way, especially early in the operational breeding season. However, the majority of veterinarians responding to a 1988 questionnaire had abandoned this practice and had shifted to early detection of twin pregnancy as a more efficient management technique.¹⁵

A breeding program that minimizes twin pregnancy but maximizes reproductive efficiency begins with breeding of all mares regardless of number of ovulatory follicles. Breed and other risk factors for twinning should be considered in designing such a program, as well as the intensity and goals of the breeding operation. In addition, a strategic management program should schedule timely examination of mares to detect twin pregnancy, prevent development of twin embryos to twin fetuses, and allow mares that have suffered abortion (spontaneous or induced) to be rebred within the same breeding season. Although ultrasonographic examinations to diagnose twin pregnancy may be less cost-effective for mares at low risk for multiple ovulations, every client should be informed of the possible consequences of undetected twin pregnancy.