Twin Pregnancy in Horses

Carrying twins to term is a severe health risk, both for the dam and the foals. In most twin pregnancies, one of the twins often naturally aborts. If both fetuses survive, they are often born smaller, and are more prone to severe, life-threatening illnesses such as septicemia.

Birthing twins carries its own severe health concerns. Both foals attempting to leave the birth canal at the same time causes dystocia, difficulty giving birth.

Dystocia can have life-threatening consequences for the mare, including uterine wall tears or rupture of arteries. Dystocia also delays the foals’ access to oxygen, which may result in death of the foals. If delivery is successful, twin pregnancies have a higher risk of retained placenta, which can lead to severe laminitis in the mare.

It is essential to have a vet examine a pregnant mare within the first month of conception to identify a twin pregnancy and proceed with treatment.

Twin pregnancies occur more commonly in Thoroughbreds, and in mares which have produced twins before.

It is essential to routinely examine pregnant mares following breeding. Mares with a history of twinning require closer monitoring, as they are more likely to develop a twin pregnancy again.

It is common for one or both foals to abort during pregnancy.

Mares are only designed to safely carry a single foal to term. Most twin pregnancies arise from a mare ovulating multiple eggs during conception, producing non-identical twins.

When both embryos settle in the same uterine horn, one often receives more nutrients than the other, outgrowing or absorbing its twin early on. If not absorbed, the weaker twin usually aborts later in the pregnancy.

Embryos developing in different uterine horns may survive longer, although one is likely to displace the other as they grow larger. These types of twins are often aborted late in gestation, as they do not interfere with each others’ growth until they are more fully developed.

There are no symptoms to differentiate a twin pregnancy from an individual pregnancy. Rectal examination of the uterus during the first month of pregnancy is critical for identifying twins.

Diagnostic imaging identifies a twin pregnancy, including transrectal or transabdominal ultrasounds.

Reducing a twin is essential for the continued health of both the mare and foal. If diagnosed early enough in pregnancy, reduction of one twin is accomplished by a ‘twin pinch’, where the embryo is crushed. This method can only be used between 9 and 15 days of gestation. Later in gestation, treatment may require craniocervical dislocation or an intracardiac potassium chloride injection.

Mares may require additional supportive medication, such as anti-inflammatories or supplemental progesterone to help maintain the pregnancy of the remaining twin. Following treatment, routine monitoring of the remaining twin is recommended to ensure its viability and continued growth.

Prognosis for aborting one twin and retaining the other is good early in pregnancy, when treated with a “twin pinch”. The prognosis is guarded past the first month, as attempting to reduce twins at a later stage of gestation often results in abortion of both fetuses.

Prognosis is poor for both mare and twins if carried to term.

There is no method of preventing a twin pregnancy.

Twin pregnancy is not contagious.

Twin pregnancies are common in horses.

Abortion of a twin

• ‘Twin Pinching’
• Craniocervical dislocation
• Intracardiac potassium chloride injection Medication
• Anti-inflammatories
• Progesterone