Equis ISSN 2398-2977

Uterus: inertia

Synonym(s): Uterine atony, absence of effective uterine contractions during labor

Contributor(s): Philippa O'Brien, Christopher Phillips


  • Uterine inertia is a rare complication of foaling in the mare.
  • Cause: it can occur as a primary condition, where reduced uterine contractility is caused by maternal debilitation, hydroamnion or hydroallantois, or secondary to myometrial fatigue following dystocia, equine protozoal myopathy.
  • Signs: weak contractions, retained fetal membranes, exhaustion.
  • Diagnosis: based on client history and the clinical signs of lack of normal expulsive efforts of the mare during foaling.
  • Treatment: the primary aim of treatment is safe delivery of the foal, using controlled vaginal delivery or caesarean section. Such foals may then require specific treatment and intensive care if delivery was prolonged. In certain situations, uterine inertia may be predicted, allowing preventative measures to be undertaken.
  • Prognosis: depends on cause.




  • Effective smooth muscle contraction of the myometrium can be adversely affected by several conditions in the mare.
  • Contraction of smooth muscle is regulated by cytosolic ionized calcium; it is thought that hypocalcemic mares and debilitated mares with metabolic derangements lack sufficient available calcium to produce effective myometrial contractions.
  • These effects can be exacerbated by previous endometrial infection.
  • A study of draft mares showed that those with retained fetal membranes   Placenta: retained  had lower serum calcium levels than those that did not, although the physiologically active ionized form of calcium was not measured.
  • Not all areas of the myometrium may be affected in the same way, resulting in tight rings of contracted smooth muscle and ineffective contractions.
  • Myometrial fatigue results from prolonged dystocia or twin delivery.
  • Mares that have undergone dystocia   Reproduction: dystocia   or caesarean section   Uterus: caesarean section  are predisposed to retained fetal membranes   Placenta: retained  .
  • This may be partly due to uterine inertia, but also because endometrial inflammation and hemorrhage caused by physical manipulations predisposes to chorionic adhesion to the endometrium.
  • Chronic overstretching of the myometrium caused by hydroamnion or hydroallantois   Uterus: hydroallantois / hydroamnios  , prevents effective myometrial contraction.
  • Uterine rupture may be a cause or consequence of inertia.
  • Uterine rupture occasionally occurs pre-term but more commonly at delivery.
  • It may be a result of vigorous fetal hindlimb extension during delivery or as a result of overzealous manipulations during dystocia   Reproduction: dystocia  .


  • Depending on the initiating cause:
    • Hydrops conditions develop rapidly over days or weeks between 6 and 10 months of gestation.
    • Debilitating systemic illness may be present from the beginning of pregnancy.
    • Dystocia can result in myometrial exhaustion after only 30 min of abdominal straining.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • LeBlanc M M (2008) Common peripartum problems in the mare. J Equine Vet Sci 28 (11), 709-715 VetMedResource.
  • Christensen B W et al (2006) Management of hydrops amnion in a mare resulting in birth of a live foal. JAVMA 228 (8), 1228-1233 PubMed.
  • Sevinga M, Barkema H W & Hesselink J W (2002) Serum calcium and magnesium concentrations and the use of a calcium-magnesium borogluconate solution in the treatment of Friesian mares with retained placenta. Theriogenology 57, 917-941 PubMed.
  • Hassel D M & Ragle C A (1994) Laparoscopic diagnosis and conservative treatment of uterine tear in a mare. JAVMA 205 (11), 1531-1536 PubMed.

Other sources of information

  • Frazer G (2011) Dystocia Management. In: Equine Reproduction. Eds: McKinnon A O, Squires E L, Vaala W E & Varner D D. pp 2479-2496.
  • Threlfall W R (2011) Retained Fetal Membranes. In: Equine Reproduction. 2nd edn. Eds: McKinnon A O, Squires E L, Vaala W E & Varner D D. Wiley-Blackwell. pp 2520-2529.
  • Blanchard T L & Macphereson M L (2007) Postparturient Abnormalities. In: Current Therapy in Equine Reproduction. Eds: Samper J C, Pycock J E & McKinnon A O. pp 465-475.