Twin conceptions are relatively common in mares and result from double ovulations of follicles from the ovaries. Both eggs are fertilized by semen in the uterus and two embryos form. They move independently around the uterus for about 16 days after fertilization then attach to the wall of the uterus. Twinning is more common in Thoroughbred and Warmblood mares and in young to middle aged mares.
About 50% of twin pregnancies naturally reduce to one fetus, a singlet. Often, one embryo resorbs (disappears) without incident. This natural reduction is more common when twins are implanted in the same uterine horn than when they are in different horns.
If both pregnancies survive into mid to late gestation, often the smaller fetus becomes distressed, causing premature labor and abortion of both pregnancies. This usually happens between 7-10 months of gestation. In rare cases, twins are born full term, alive, and survive.
The standard diagnostic for this diagnosis is veterinary trans-rectal ultrasound at 14-20 days gestation. Twin embryos can be confused in this early ultrasound with uterine cysts. If there is any doubt about the presence of twins, repeat ultrasound exam may be needed. Twins can also be detected later in pregnancy either by rectal palpation or more commonly ultrasound, but the diagnosis is much more difficult and there is little one can do other than terminate the entire pregnancy.
When diagnosed early, one of the embryos can be eliminated, allowing the other to survive.
Other Diagnoses Considered
Treatments May Include
Prognosis & Relevant Factors
The prognosis is good if a twin can be eliminated by about day 25. After that, it becomes increasingly harder to reduce twins without terminating the entire pregnancy.
Mares that give birth to twins are at risk for retained placenta and uterine infection.
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