Conditions or ailments that are the cause of a problem that you see - your observation.

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Twin Embryos or Fetuses Detected on Ultrasound


Twin conceptions are surprisingly common in mares, and result from multiple ovulation of follicles from the ovaries. Both eggs are fertilized by semen in the uterus, and two embryos form. The twin embryos move independently around the uterus for about 16 days after fertilization, and then attach to the wall of the uterus.

Twinning is more common in Thoroughbred, Warmblood and Draft Horse mares and in young to middle aged mares.

About 50% of twin pregnancies naturally reduce to one fetus, called 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 test for twins 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.

TREATMENT- The most common and effective method for reduction of twins to a singlet is manual, trans-rectal reduction of the embryos through crushing, usually under ultrasound guidance.

my vet's role


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 likely to abort. If they do go to near term or term, then retained placenta and uterine infection are relatively common. In most cases, one or both twin foals are born dead. if they are born alive, they tend to be small and somewhat stunted.

my role

Questions To Ask Your Vet:
  • What method can be used to reduce twins to a singlet in this case?
  • What are the chances of successful reduction to one fetus?
  • Are you sure that these are twins, and that this is not a uterine cyst?

Recognize how common twin pregnancies are and understand the reasons why they are undesirable.

Your vet should perform an ultrasound at day 14-16 post-ovulation, searching for twins. A second ultrasound should be performed at between 40 and 50 days post-ovulation to ensure that the surviving singlet pregnancy is alive and well.

Mares that have had twins will likely twin again. Your vet will check and double check mares with that history to be sure there are not twins.

further reading & resources

Author: Doug Thal DVM Dipl. ABVP