Loss of Pregnancy

Embryonic loss is loss of the embryo before 45 days of pregnancy. Early loss of the pregnancy usually goes undetected, unless the mare is checked regularly by ultrasound during the pregnancy. The cause of embryonic loss in mares is not always easy to ascertain but it is often associated with endometrial changes in older mares (periglandular fibrosis, cystic glandular distension) or endometritis in mares of all ages, or problems specific to older mares, such as uterine diverticula, or aged oocytes (such as it happens in women).

Fetal loss or abortion is defined as that pregnancy loss occurring after 45 days of pregnancy. It is usually undetected, however, until after 5 to 6 months of pregnancy, when the fetus has acquired considerable size. Common causes of abortion in mares include twinning, rhinopneumonitis and other viruses, like equine viral arteritis. Adequate management and vaccination for rhinopneumonitis may prevent abortion in your mare.

Twinning

Twinning is undesirable in mares. If mares become pregnant with twins it usually results in late term abortion of both fetuses before they are mature and developed enough to survive due to lack of sufficient room and nutrition for both foals in the uterus. Twins in horses are always the result of double ovulations, or fraternal twins. Therefore, it is important to record double ovulations if mares are being monitored by an internal exam during breeding management. Whenever double ovulations are suspected or recorded, a thorough pregnancy exam should be performed at 12 and 14 days after ovulation.

Early detection of twins allows elimination of one of the embryos very easily, by pinching it through the rectal and uterine walls during ultrasound exam. If twins are detected later in pregnancy then the only alternative may be to induce abortion, or wait/hope for spontaneous reduction/death of one of the embryos and survival of the other one.