Reproductive anatomy + pathology

Equine Female reproductive tract

3 ovary stages

Oviduct

• Infundibulum: made up of many fimbriae (little fingers) that surround the ovary, collecting the ovulated egg

• Isthmus (4): “tortuous”, narrow tube that the ovulated egg travels through to get to the uterus

  • Where fertilization occurs (egg + sperm)

• Junction of the isthmus and uterus only allows fertilized eggs to enter the uterus (5)

recognition of pregnancy

• Fertilized egg makes it to the uterus ~6 days after fertilization

• The conceptus is very mobile, and we believe the movement and frequent contact with the endometrium (inner lining of the uterus) aids in Maternal Recognition of Pregnancy

  • Prevents Corpus Luteum from regressing on the ovary, which maintains pregnancy

Placentation

• The equine conceptus is highly unusual in that it does not form a blood connection with the mare until around 40 days of pregnancy, & takes until day ~150 to complete

• Placental attachment to the uterine wall is 

  • Epitheliochorial: The chorion portion of the fetal membranes is in contact with the mare’s uterine lining

  • Diffuse: The chorion has contact with the uterine lining all over, not only in specific locations

• 3 membranes: amnion, allantois, chorion

Gestation

• Pregnancy is diagnosed either by hand via rectal palpation or by ultrasound

  • By hand – 25-30 days of pregnancy

  • Ultrasound – 11-14 days of pregnancy, fetal heartbeat by day 28

• Mares are pregnant for 335-340 days

  • Range from 300-385

  • Can be longer if the mare has a low plane of nutrition, in cooler weather, and if carrying a mule as opposed to a foal

Parturition

• Act of giving birth: “Foaling”

• Signs of foaling include:

  • Udder distension: 2-6 weeks prior

  • Relaxation of muscles & ligaments around the pelvis: 7-10 days prior

  • “Waxing”: 2-4 days prior

  • Separation from the herd: in the hours prior

stages of parturition (3)

1. Abdominal discomfort, restlessness, uterine contractions

• Fetal positioning

2. Begins with fetal membrane rupture & release of amniotic fluid – “water breaking”

• Abdominal contractions, birth of foal

• Typically 15-30 minutes

• Amniotic membrane should break when born 

3. Passing of fetal membranes

• Within 3 hours

Placentitis

• Vaginal discharge & premature “bagging up”

• Ascending – infectious organisms travel up the repro tract

  • Poor vulvar conformation

• Hematogenous – where bacteria circulate through the bloodstream

• Placenta can be affected diffusely (all over) or localized infection

• Can lead to detaching of the placenta or can lead to issues at parturition & can be difficult to break

• Check routinely throughout pregnancy via ultrasound

Hippomanes

• Amorphous pieces of allantoic material that is often delivered along with the fetus

• Form like an enterolith – fluid and particulates collect around a central nidus

• Mostly comprised of mucoproteins and calcium phosphate

• Are typically beige-brown-green in color, firm, and up to several centimeters in diameter

• Not pathologic

Twins

• Terminate one (or both) if <35 days

  • Pinch

• Issues with twins

• Rarely both born full term, & alive

• Likely to have retained membranes

• Likely to have a dystocia

Premature Chorioallantois Separation

• Red Bag

• Outer fetal membranes (chorion + allantois) separate from the uterine lining before the amnion ruptures

• Can quickly result in the foal not getting enough oxygen & dying via suffocation

• True emergency

• Caused by placentitis, twins, fescue toxicity, or for no reason at all

Dystocia

• Foaling problem that does not allow the mare to give birth on her own

• Mare causes vs fetal causes

• Serious problem, often fatal for the mare &/or for the foal

• Must consider cleanliness as well as gentleness when assisting a mare

• The vulva, vagina, and uterus can be easily traumatized

• Most should be addressed by a veterinarian or an experienced caretaker

Mare + Foal causes of dystocia

Mare causes

• Uterine atony – failure of the uterine muscles to contract

• Uterine torsion – uterus that has twisted on its axis, cranial or caudal to the cervix
  

Foal causes

• Malposition

• Fetal malformation

Addressing a Dystocia in a Standing Mare

• Clean the external vulva and surrounding tissue well with iodine and warm water

• Cleanliness is essential to prevent infection of the mare’s reproductive tract

• There is no such thing as too much lubrication!

• General rule is that if traction (pulling) requires more strength than that of two strong men, an alternative method should be considered

• Often requires the use of obstetrical chains to aid in grip and traction

Addressing Dystocia: secondary options

• Controlled Vaginal Delivery

  • Mare must be anesthetized

  • Hoist the mare’s hindquarters to push the GI tract cranially

• Cesarean Section

  • Delivery of the foal by an abdominal and uterine incision

• Fetotomy

  • When cesarean section is not an option

  • Fetus is cut into manageable pieces within the uterus, to be delivered vaginally with assistance

Examining placenta

• Must examine the fetal membranes after every delivery

• Normal birth: fetal membranes ruptured by the foal in “cervical star” – a weakened area of the chorioallantois

• Should appear as the letter “F”, representing the two uterine horns and the uterine body

Retained placenta

• Any missing pieces that have remained in the uterus, or if the entirety of the fetal membranes have not passed within 3 hours, the membranes are considered to be retained

• Can be treated with oxytocin administration to promote uterine contractions in early cases

• Must be treated with antibiotics and anti-inflammatories in cases >8 hours

• Predisposes mares to laminitis