ISSN 2398-2977      

Abortion: early embryonic/fetal death


Synonym(s): EED, EFD


  • Early embryonic death (EED) defined as death of the conceptus before 40 days gestation:
    • Mares which end the season barren having been scanned pregnant but lost embryo, or mares which were never scanned pregnant, but did in fact conceive.
    • Divide into three periods:
      • <6 days.
      • 6-12 days.
      • 12-42 days.
  • Early fetal death (EFD) defined as death of the conceptus after 40 days gestation:
    • 0-40 days - 8-17%
    • 12-20 days - c 10%.
    • 21-30 days - c 4-7%.
    • 31-40 days - c 6%.
  • Cause: poorly defined; twinning, malnutrition, endometritis, endotoxin; general categories are genetic, environmental and endocrine (?insufficient maternal progesterone to maintain pregnancy).
  • Signs: return to estrus (variable period); decreased pregnancy rates; pseudopregnancy.
  • Diagnosis: rectal examination, ultrasonography, biochemistry.
  • Treatment: hormone therapy; treatment for identified cause.
  • Prognosis: depends on cause.



  • Inadequate endogenous progesterone production by corpus luteum or placenta:
    • Low serum concentrations of progesterone associated with early embryonic loss in some mares.
    • Conflicting evidence in the literature relating to whether a primary luteal deficiency is a significant cause of embryonic death in normal cycling mares in the breeding season (lack of gonadotrophin releasing hormone in seasonally anestrus mares may result in embryonic loss if those mares are stimulated to ovulate).
  • Twinning   Twinning  .
  • Malnutrition:
    • Maternal malnutrition between 25 and 31 days gestation.
    • Temporary starvation has been used in an attempt to encourage the reduction of adjacent twin concepti   Twinning  to a singleton with variable success.
  • Bacterial endometritis:
  • Endotoxin-induced prostaglandin release - early and late pregnancy.
  • ?Failure of maternal recognition (small embryo).
  • Genetic factors:
    • Inbreeding.
    • Ageing in sperm/ovum.
  • Age of mare:
    • Pregnancy rates in young and old mares at day 2 similar, but by day 4 much greater loss rate in older mares.
    • Also increased incidence of loss 14-42 days in older mares.
    • Combined effect of oocyte deficits; reduction in embryo quality; ?Role of endometrium.
  • Stallion factors:
    • Chromosomal defects in early embryogenesis.

Predisposing factors

  • Maternal age: a decline in fertility with concomitant increase in EED/EFD rate is seen in mares in middle to late teens.
  • Used to be thought that chronic degenerative endometrial disease was the major contributing factor.
  • Recent evidence shows that when morphologically normal embryos were transferred into normal and aged mares with endometrial degeneration there was no difference in pregnancy rates at 12 or 28 days, ie endometrium degeneration not the cause of EED.
  • Recent evidence shows that transfer of embryos from aged mares to young recipients results in lower pregnancy rates than embryos from young donors transferred into young recipients, ie the embryos of aged mares are defective.
  • Further experimental evidence shows that when oocytes from young and aged were transferred into the oviducts of young recipients there were significantly reduced pregnancy rates using oocytes from aged mares, ie oocytes of aged mares are defective and contribute significantly to reduced fertility/EED.
  • Mares bred at foal heat (first post-partum estrus) have higher incidence of EED/EFD.
  • Low grade endometritis   Uterus: endometritis - bacterial  , pneumovagina   Vagina: pneumovagina  .


  • Many causes suggested, few established.

Recurrent endometritis appears to accelerate chronic degenerative endometritis and produce diffuse stromal fibrosis.

Genetic factors

  • May be inherited or occur at fertilization.

Environmental factors

  • Acute malnutrition.
  • Poor body condition.
  • Severe stress - temperature, colic, dehydration.
  • Infection - lowgrade, non-specific bacterial/fungal.
  • Hostile uterine environment.
Loss of embryonic vesicle
  • Normal rates of fertilization in young mares are between 91-96%; in older subfertile mares between 81-92%.
  • Most subfertility occurs before day 7-10 after fertilization: losses of 25% and 60% occur to young fertile and old subfertile mares respectively.
  • Reports of losses between days 12 and 50 vary widely.
  • Vesicles <4 mm at 13 days, or not visible until day 14 or later, after ovulation are likely to fail possibly due to:
    • Failure of antiluteolytic signal.
    • Mare returning to estrus and vesicle aborted.
    • Failure to develop an embryo and aborting after 30 days.
  • Undersized vesicles only survive in 3% of mares that retain their pregnancies.
  • Normal sized vesicles can be first seen on day 11-13 after ovulation and grow at a rate of 1-4 mm per day. By day 17/18 diameters can reach 20-30 mm.
  • Vesicles are usually spherical but occasionally ovoid up to day 16/17 after ovulation then progressively more irregular; shape does not appear to influence viability.

Loss of embryo

  • Bacterial endometritis:
    • Low grade persistent acute endometritis   Uterus: endometritis - bacterial  established before, or at mating with inadequacy of uterine defense and drainage mechanisms   →   infection of the fetus or following fetal membranes.
    • Chronic degenerative endometritis   Endometrium: fibrosis    →   endometrial gland and/or placental dysfunction.
  • Endotoxin:
    • Pyrexia, stress or septicemia   →   circulating endotoxin   →   increasing levels of PGF2-alpha   →   decreased progesterone   →   luteolysis (most sensitive period up to day 45).

>Day 45 additional progesterone is provided by supplementary corpora lutea.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • McKinnon et al (2000) The inability of some synthetic progestagens to maintain pregnancy in the mare. Equine Vet J 32, 83-85 PubMed.
  • Newcombe J R (2000) Embryonic loss and abnormalities of early pregnancy. Equine Vet Educ 12, 88-101 VetMedResource.
  • Barbacini S, Gulden P, Marchi V & Zavaglia G (1999) Incidence of embryo loss in mares inseminated before or after ovulation. Equine Vet Educ 11 (5), 251-254 Wiley Online Library.
  • Gilbert R O & Marlow C H B (1992) A field study of patterns of unobserved foetal loss as determined by rectal palpation in foaling, barren and maiden Thoroughbred mares. Equine Vet J 24 (3), 184-186 PubMed.
  • Woods J, Bergfelt D R & Ginther O J (1990) Effects of time of insemination relative to ovulation on pregnancy rate and embryonic-loss rate in mares. Equine Vet J 22 (6), 410-415 PubMed.

Other sources of information

  • Budik S et al (2006) Effects of Progesterone Treatment during the Luteal Phase on Quantitative Gene Expression in Equine Embryos Day 10 Post-Ovulation. In: Proc ISER IX. pp 366.
  • Ball B A (2000) Ed. Redcued Reproductive Efficiency in the Aged Mare: Role of Early Embryonic Loss. In: Recent Advances in Equine Reproduction. International Veterinary Information Service, Ithaca NY. A0201.0300. Website: Last updated 30th March 2000.

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