Twins are a significant cause of reduced reproductive efficiency in the mare and may result in early pregnancy loss, abortion, stillbirth, or early neonatal death. It is estimated that less than 10% of twin pregnancies make it to term with two live healthy foals born alive.
Origin of Twins
Genetically identical or monozygotic twins result from fertilization of a single oocyte (egg) by a spermatozoon and the resultant embryo splits into two separate embryos. Non-identical or dizygotic twins are the result of fertilization of two oocytes ovulated from separate follicles. In the horse, non-identical twins are much more common than identical twins.
Sperm from a fertile stallion can survive in the reproductive tract of a mare for several days. Twins may therefore develop from ovulations that occur on the same day (synchronous ovulations) or several days apart (asynchronous ovulations). Thoroughbreds, Warmbloods, and Draft horses have the highest incidence of multiple ovulations and twins. Arabians and Quarter Horses have a lower rate of double ovulations and consequently have a lower overall incidence rate of twins. Individual mares tend to ovulate the same number of follicles per cycle. Consequently, mares that double or triple ovulate or have had twins in the past should be monitored closely for multiple ovulations on subsequent breeding cycles. In addition, administration of an ovulation induction agent such as human chorionic gonadotropin (hCG) or deslorelin acetate is associated with a higher rate of multiple ovulations and consequently an increased risk of twins.
Outcome of Twins
Natural resorption or elimination of one embryo may occur if two embryos are adjacent to one another in one uterine horn (i.e., unilateral twins). Natural reduction is less likely to occur if the embryos are located apart or in opposite uterine horns (i.e., bilateral twins). If a twin pregnancy does make it past 35 to 50 days of gestation, it is common for abortion to occur between 7 and 9 months. The primary cause of abortion in mid-gestation twins is placental insufficiency. However, if one twin is located in each uterine horn and their placentas equally “share” the uterine body, some twin pregnancies can make it to term and result in the birth of live foals. A more common scenario is that equine twins do not equally share space in the uterus. Typically, one fetus is relegated into the upper part of one uterine horn and has limited room for normal placental-endometrial contact. As a consequence, there is insufficient nutrient and gas exchange to maintain normal fetal development. The other fetus is usually located in the opposite uterine horn and its placenta has endometrial contact throughout the entire uterine horn plus the uterine body. This second fetus generally has sufficient nutrition to maintain reasonably normal growth and development. Unfortunately, inadequate placental support eventually results in the demise of the growth-restricted fetus and its death results in abortion of both fetuses. Occasionally, one fetus will die in utero and be retained as a mummified fetus along with a normal viable developing fetus. In this scenario the mummified fetus is delivered along with the viable fetus at term.
Reproductive Management of Twins
Good breeding management practices, anticipation of the potential for twins and an early accurate pregnancy diagnosis are keys to successful management of twins. Twin pregnancies can be diagnosed with transrectal ultrasonography as early as 11 to 12 days after ovulation. Management of twins is most successful 14 to 16 days post-ovulation. Equine embryo(s) “migrate” throughout the uterus via uterine contractions until about day 16 after which they “fix” in one location. It is highly advantageous to identify twins during the mobility phase, so that one embryo can be eliminated when it is in a separate location within the uterus from the other embryo.
“Unilateral twins” are located in the same uterine horn and sometimes are in direct contact with each other. It may be necessary to re-examine the mare 1 to 2 hours later or the next day when the embryos have had a chance to separate naturally due to uterine contractions. Alternatively, gentle controlled manipulation with the ultrasound probe can be used to separate unilateral twins.
“Bilateral twins” are located in opposite uterine horns. In this scenario, one embryo should be reduced immediately upon detection. There are several well-described techniques that can be used to successfully disrupt one embryonic vesicle. One common technique is to use the ultrasound probe to apply gentle pressure on one embryonic vesicle to disrupt the outer membranes. A manual technique using the fingertips can also be utilized to disrupt one embryonic vesicle.
Reduction of one embryo allows for an opportunity for the remaining viable embryo to continue development and hopefully result in the birth a live healthy foal.