Pregnancy loss

Pregnancy failure can be divided into two categories:

  1. Early Embryonic Loss
  2. Abortions

Early Embrionic Loss

Death of the conceptus 15-50 days after ovulation has been estimated to be 5-24%.

Maternal factors
  • Endocrine – low progesterone production. This can be due to failure of maternal-foetal recognition or deficiencies in ovarian progesterone production
  • Oviduct environment – improper timing of insemination so the ovulated egg and sperm do not meet at the correct place.
  • Uterine environment – endometritis, fibrosis, intraluminal fluid accumulation, large endometrial cysts. Any problem with the previous pregnancy such as a difficult birth or retained placenta can adversely affect the uterine environment in a subsequent pregnancy
  • Mare age – age related degeneration of the uterus and viability of the oocyte
  • Foal Heat mating – pregnancies established at the foal heat have a reduced viability and reduced conception rates
  • Twin pregnancies
Embryonic factors
  • Genetic factors e.g. chromosomal abnormalities.
  • Small size, morphological defects.
Environmental factors
  • Stress reduces progesterone production
  • Malnutrition of the mare and over or underfeeding in the period after mating can adversely affect her fertility
  • Any illness resulting in a high temperature or prolonged colic

Early embryonic loss may be predicted from the appearance on the ultrasound scan. The signs include:

  • An embryonic vesicle that is smaller than expected for its age
  • A vesicle with no embryo visible after the time when it should be seen
  • An embryo that is smaller than expected for its dates


Abortion is defined as the delivery of a dead foetus less than 320 days into gestation i.e. termination of pregnancy before the foal can survive outside the uterus. The overall rate of abortion in the horse population varies from 5-15%. If abortion occurs before 4 months the abortion is rarely noticed as the foetus is too small. Abortions can be caused by infectious or non-infectious factors.

  • Equine Herpes Virus (EHV) – usually abort late pregnancy i.e. over 7 months. Mare shows no symptoms. Can cause abortion ‘storms’ leading to in contact mares also aborting in same stage of pregnancy.
  • Equine Viral Arteritis (EVA) – usually abort in early pregnancy i.e. less than 3 months. Mares are usually sick and show symptoms of infection.
  • Placentitis (infection of the placenta) – bacterial or fungal infections of the placenta. This is the most common cause of abortions. Common infections are Pseudomonas Aeruginosa, Streptococcus Zooepidemicus, E.Coli, and Aspergillus Fumigatus.
  • Systemic infections/toxaemia.
  • Equine Infectious Anaemia (EIA)- abortions occur in last half of gestation. Low incidence rate.
    • Twins – see below
    • Progesterone deficiency
    • Umbilical cord abnormalities, e.g. strangulated or twisted cord
    • Stressed mare
    • Malnutrition
    • Uterine torsion
What should I do if my mare aborts?
  • Isolate mare – she may be infectious and may be a risk to other mares that are in foal
  • Retain the aborted foetus and placenta (afterbirth) for the vet to examine
  • Call the vet
What will the vet do?
  • Take a history – the vet will ask a number of questions relating to the mare’s reproductive history e.g. stage of pregnancy, vaccination status, number of in-foal mares on the premises, health of mare, previous foaling history, clinical signs of mare before or after mare aborted, general feeding and management of mare. This will give clues to the reason for the abortion and will also be needed to forward to the lab with samples of the aborted foetus.
  • Examination of mare – full clinical exam will determine whether the mare is sick and requires treatment. Internal exam will be carried out to check for uterine tears, retained placenta, endometritis and presence of a second foal (twins).
  • Examination of aborted foetus and placenta – the appearance of the fetus and placenta can provide vital clues to the cause of the abortion e.g. fungal infections produce white plaques within the placenta, EHV infections produce jaundiced foetus with necrotic spots within the liver. Full examination of the placenta will ensure that there is no retained placenta still present within the uterus. Retained placenta is commonly seen in cases of abortion.
  • Submit samples to the diagnostic laboratory- the vet may decide to send the entire foetus and placenta to the lab, or may take a number of samples of the fetus including stomach and stomach contents, liver, lung, kidney, spleen, adrenal gland, placenta and uterine fluid.

The results of these examinations and tests will be vital, not only in diagnosing the cause of the abortion, but for recommending the correct treatment for the mare, risk assessment of other mares on the premises and also predicting the breeding future of the affected mare.


Twins are highly undesirable in the mare. If twins are not identified and treated in early pregnancy the usual outcome is abortion in late gestation. Nearly all twin pregnancies result from double ovulations. These may occur close together or several days apart within a single oestrous period. The incidence of double ovulations differs between breeds and increases with the age of the mare. Thoroughbred mares are reported to have a 15 – 25 % occurrence of double ovulations with a 15% incidence of twins.

Twinning is a serious source of loss to the breeding industry. Various reports suggest that between 53 and 73% of affected mares will abort and of all the mares with twin pregnancies, only 16-25% of mares will give birth to single or twin foals.

The cause of embryonic loss is the inability of the endometrium to provide adequate nutrition for both embryos. Twins located in the same horn are likely to die earlier in the pregnancy because the vesicles are in contact with each other rather than with the lining of the uterus and their nutrition is reduced.

Complications that can also arise with late term abortion or attempted delivery of twins include dystocia (difficulty giving birth), retained placenta, metritis and of course death of the twins. If twins are successfully delivered, they are usually underweight and require a great deal of intensive care. It is common for one or both of them to die within a few days of birth. Those that survive are less likely to achieve a high level of performance than single foals.

Diagnosis and treatment of twins

Before day 35 of gestation:

Twinning is diagnosed by routine ultrasound examination per rectum. The best time to diagnose twins is at day 14-16 of gestation. At this stage, the conceptuses have not attached to the uterine wall and are also still small enough to manipulate. Usually the smaller of the conceptuses is gently moved away from the larger conceptus to the tip of the uterine horn, and is then squeezed between the ultrasound probe and endometrium until a ‘pop’ is felt. This can be a complicated procedure and should only be carried out by a vet. With experience, success rates can be as high as 90%. After day 17, the conceptuses attach to the endometrium, making it much more difficult to separate and crush one of the twins. Manual crushing of the twins becomes progressively less successful the longer the pregnancy is allowed to continue. Twins can be successfully squeezed up to 35 days gestation.

After day 35 of gestation:

Manual squeezing of one twin is impossible after day 35 and two options are available:

Very experienced stud vets may attempt to eliminate one of them by guiding a sterile needle through the wall of the vagina and into the foetal sac under ultrasound guidance.

Prostaglandin injections to abort both twins may be required after day 35. Around 35 – 40 days, endometrial cups have developed and are producing progesterone to maintain the pregnancy. Therefore, prostaglandin will cause abortion but will not remove these endometrial cups; hence the mare will still think she is pregnant even though the twins have been aborted. The mare will then not come back into season until the following breeding season.

Two pregnancy scans at days 14 – 16 then again at days 25 – 30 are imperative to successfully diagnose and treat twins without significant complications.

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