Equis ISSN 2398-2977

Uterus: hydrops/hydroallantois/hydroamnios

Synonym(s): Hydrops amnii (hydramnios), Hydrops allantois (hydrallantois, dropsy, hydroamnion)

Contributor(s): Wendy E Vaala, Elaine Watson, Madeleine L H Campbell


  • Cause: a rare condition where excessive fluid within the allantois or amnion of the pregnant mare accumulates; often associated with severe fetal abnormalities.
  • Signs: (rapid) abdominal enlargement, abdominal pain +/- spontaneous abortion; dyspnea, anorexia.
  • Diagnosis: rectal examination, ultrasonography.
  • Treatment: induced abortion +/- assisted delivery of foal.
  • Prognosis: fair to hopeless for foal; guarded for mare.



  • Unknown.
  • Probably multifactorial.
  • Possible hereditary link.
  • Associated with other abnormalities of pregnancy - fetal deformities Hydrocephalus, multiple pregnancies Twinning, placentitis and placental insufficiency.

Predisposing factors


  • Congenital deformities in the foal.
  • Placental dysfunction/insufficiency.


  • Possible hereditary predisposition.


  • Hydroallantois may be associated with structural or functional changes in the chorioallantoic membranes, eg dysfunction of ion pumps.
  • Placental dysfunction/insufficiency results in excessive production or imbalanced production/removal of allantoic fluid (amniotic is extremely rare) with the results that the mare's health suffers from the excessive weight of fluid, respiratory compromise, dehydration and/or gastrointestinal dysfunction.
  • In cases of hydroallantois, fetal hydronephrosis has been reported, but it is unclear whether this is a contributing cause or is being caused by the hydroallantois.
  • Alteration in fetal/placental maternal circulation → abnormal accumulation of allantoic fluid → fetal compromise and death.
  • Large physiologic variation in quantities of fluid in mares - usually quantities of >20-25 liters are involved in clinical cases.
  • In cases of hydroamnios congenital abnormalities which prevent swallowing and recycling of amniotic fluid by the fetus result in accumulation of amniotic fluid to the point at which the mare's health is compromised.
  • In both hydroallantois and hydroamnios, maternal edema → compression of epigastric veins → exacerbates edema → stress on prepubic tendon or abdominal muscles → rupture.
  • Abdominal pressure can impinge on the diaphragm resulting in labored breathing, discomfort and an increased pulse rate.
  • Severe ventral edema can build up which together with the increased abdominal size can → difficulty in walking and even recumbency.
  • Ultimately, in both hydroallantois and hydroamnios, endocrine factors initiate spontaneous abortion → uterine atony/interia → failure to expel fetus → fetal autolysis.


  • Between 6-10th month's of gestation.
  • Rapid onset over days to a few weeks.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Crabtree J (2012) Peripartum problems in mares. 1. Prepartum problems. In Pract 34 (7), 400-410 VetMedResource.
  • Elliott C & Mitchell A (2012) Hydroallantois in a recipient mare. Vet Rec 171 (7), 177 BMJ.
  • Troedsson M H T (2007) High risk pregnant mare. Acta Vet Scand 49 (Suppl 1), 89 VetMedResource.
  • Christensen G W et al (2006) Management of hydrops anmion in a mare resulting in birth of a live foal. JAVMA 228 (8), 1228-1233 PubMed.
  • Perkins N R & Frazer G S (1994) Reproductive emergencies in the mare. Vet Clin North Am Equine Pract 10, 643-670 PubMed.
  • Sertich P Let al (1994) Hydrops amnii in a mare. JAVMA 204, 1481-1482 PubMed.
  • Giles R C et al (1993) Causes of abortion, stillbirth, and perinatal death in horses - 3,527 cases (1986-1991). JAVMA 203, 1170-1175 PubMed.
  • Honnas C M et al (1988) Hydroamnios causing uterine rupture in a mare. JAVMA 193 334-336 PubMed.
  • Welchli R O & Ehrensperger F (1988) Two related cases of cerebellar abnormality in equine fetuses associated with hydrops of fetal membranes. Vet Rec 123, 513-514 PubMed.
  • Vandeplassche M et al (1976) Dropsy of the fetal sacs in mares - induced and spontaneous abortion. Vet Rec 99, 67-69 PubMed.

Other sources of information

  • Waelchii R O (2011) Hydrops. In: Equine Reproduction. Eds: McKinnon A O, Squires E L, Vaala W E & Varner D D. Wiley-Blackwell, Chichester. pp 2368-2372.
  • Bain F T & Wolfsdorf K E (2003) Placental Hydrops. In: Current Therapy in Equine Medicine. pp 301-302. Science Direct (Elsevier).
  • Brown C M & Bertone J J (2001) Eds. The 5-Minute Veterinary Consule - Equine. 1st edn. Lippincott, Williams & Wilkins. ISBN: 0 683 30605 7.