Twin pregnancies in horses are common but carry significant risks for both the mare and the unborn foals. The horse’s reproductive system has developed to carry one fetus to term, and the addition of a second fetus results in competition for nutrients, oxygen and space within the womb.

In most cases, twin pregnancies end in abortion of both fetuses during mid-gestation. If the foals survive, they have an increased risk of health complications and a higher mortality rate compared to singleton foals. Mares that have given birth to twins also have an increased risk of retained placenta and traumatic injury during foaling.

The diagnosis of twin pregnancies is simple and rapid due to widely accessible ultrasound technology in veterinary clinics. In most cases, twinning is managed by terminating one of the embryos or fetuses, or by terminating the pregnancy entirely. Methods for termination include manual embryo crush (“twin pinching”), twin puncture, or craniocervical dislocation.

Early diagnosis is key for a successful twin reduction that allows survival of the remaining singleton foal. Mares should be examined between day 14 and 16 of gestation to identify the number of embryos present, as twin pinching has the best success rate at this stage.

Twinning in Horses

Twinning in horses refers to carrying and delivery of two foals instead of the usual single foal. Unlike other mammals, such as dogs or sheep, horses are not normally capable of carrying multiple fetuses successfully.

In most cases, twin pregnancy results in abortion of one or both fetuses, nullifying the time and expense invested in achieving pregnancy for breeders. Historically, twinning has been a common cause of equine abortions, representing between 20-40% of abortion cases. [1]

More widely accessible ultrasound technology has decreased the incidence of twins in horses, by allowing for rapid and easy diagnosis of twins early in pregnancy. [2] As a result, twin pregnancies now account for only 3% of equine abortions. [3]

The Normal Reproductive Cycle

Mares are seasonally polyestrous, meaning that they undergo multiple heat cycles on a seasonal schedule. Typically, mares cycle when days are getting longer in the spring and summer.

During a normal reproductive cycle, the mare’s ovaries are continuously developing follicles, structures which contain a single egg. At any given time, follicles at various stages of development can be found on a mare’s ovary.

When a follicle is fully matured, ovulation occurs, where the mature follicle ruptures and releases the egg into the oviducts. In most cases, only one follicle ovulates per estrous cycle.

Shortly after ovulation, the egg is receptive to fertilization by sperm, resulting in an embryo. Embryos are found freely floating in the uterus up until Day 16, when they begin “fixation” and start forming a placental attachment to the mare.

Fetal development continues until a minimu