Induction of Labor in the Mare: Procedures and Pitfalls

Patrick M. McCue, DVM, PhD, DACT | Colorado State University | Published: Issue 1 2024

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The goal of an elective induction of labor is to stimulate the foaling process in a full-term mare when the mare is almost ready to give birth naturally and allow for a controlled delivery a few hours earlier than her natural birth time.

Induction of foaling in mares can be very successful if strict adherence to guidelines for assessment of fetal readiness for birth is followed. However, emergency medical situations may override strict adherence to the standard guidelines in certain clinical situations.

The recommended choice for induction is repeated low doses of oxytocin. Controlled induction of labor can allow for safe predictable foaling during the day when breeding farm and veterinary medical support personnel are available.

Elective induction of labor in the mare for non-medical reasons is generally not recommended. Under most circumstances, it is advised that the mare be allowed to foal naturally without hormonal intervention. Owners should be aware that normal foaling does not occur until the fetus is physiologically mature and capable of surviving outside of the dam.

The physiologically mature fetus will send a signal to the mare to initiate the parturition process. In other words, the mare will begin labor once the fetus is ready to be born and not before. Fetal maturity is critical to survival, and early termination of a pregnancy, even at an apparently appropriate gestation length, can put a foal at risk.

One of the most common requests from owners is for intervention when a mare is one to two weeks past her ‘due date’ (i.e., the mare is 350 to 360 days or more in foal). Induction of labor in such a mare may result in the delivery of a newborn that is not physiologically mature (i.e. dysmature) and is at increased risk of several neonatal diseases. Consequently, it is not advisable to induce labor in a mare that is ‘overdue’ and not exhibiting signs that foaling is imminent.

INDICATIONS for Induction of Labor

Induction of labor does allow for an opportunity to have professional assistance available to assist with foaling and care of the newborn foal from a high-risk pregnancy.

Clinical reasons for a controlled induction of labor in a mare include management of high-risk pregnancies, monitoring of labor in mares that have had a history of dystocia or stillbirth, premature separation of the placenta (‘red-bag’), or other foaling complications.

In addition, induction of labor may make it easier to prevent a newborn foal from nursing from its dam when neonatalisoerythrolysis (jaundice foal syndrome) is expected.

Occasionally, induction of labor may need to be performed as an emergency procedure on a late-term pregnant mare with a lifethreatening medical condition, such as severe acute laminitis, ruptured prepubic tendon, placental hydrops, or other issues. In such situations, case selection criteria and recommended guidelines for induction are bypassed in an attempt to save the life of the foal and/ or mare..

GUIDELINES for Induction of Labor

Stringent guidelines should be followed for the selection of mares that are candidates for induction of labor (Table 1). To be considered for induction of labor, a mare should have a gestation length of at least 330 days based on known breeding or ovulation dates. The average duration of pregnancy in the mare is approximately 340 days, but gestation length may vary significantly between mares and ranges from 320 to 360 days.

Estimations of potential due dates must take into account season, as mares foaling during the summer have a shorter gestation than mares foaling during the winter. A candidate mare should have an enlarged udder and engorgement of the teats with colostrum, indicating that foaling is near. It should be noted that primiparous (‘maiden’) mares may not exhibit significant mammary development and colostrum production until immediately prior to parturition.

In addition, mares that have been exposed to tall fescue grass infested by the fungus Acremonium coenophialum during the last 60 to 90 days of pregnancy may fail to exhibit normal udder development and fail to produce colostrum prior to foaling.

Table 1. Guidelines for selection of mares for induction of labor

  • Minimum gestation length of 330 days
  • Mammary gland development
  • Teats engorged with colostrum
  • Elevated milk calcium levels (>200 ppm)
  • Relaxed perineal muscles
  • Elongated vulva
  • Cervical Softening
  • Waxing of teat ends

Fetal maturation is correlated with concentrations of milk electrolytes. A rise in milk calcium and potassium levels and a decrease in sodium levels are associated with increased fetal maturity. On a practical level, a mammary secretion calcium carbonate level of greater than 200 ppm is a good indicator of fetal maturity and is one benchmark for induction of labor qualification.

Mares in which natural foaling is eminent will exhibit significant relaxation of the muscles around the perineum and relaxation or elongation of the vulva. An additional criterium that may be used to determine if the mare is ready to foal is the degree of softening of the mare’s cervix as detected during a digital examination per vagina.

Mares with a dilated cervix have been reported to deliver foals more quickly than mares with a tight cervix following oxytocin administration. Dilation of the cervix and a short interval from induction of labor to delivery may be correlated to better adaptation of the neonate after birth and fewer intrapartum and postpartum complications.

Strict adherence to these guidelines for the selection or management of mares prior to induction of labor should be maintained. The equine fetus is at great risk of prematurity or dysmaturity if labor is induced at an inappropriate time.

TECHNIQUES for Induction of Labor

Numerous hormonal techniques have been tested for induction of labor in the mare. These have included the use of oxytocin, prostaglandins, and corticosteroids.

Dexamethasone

Corticosteroid (i.e., dexamethasone) administration is very effective in inducing parturition in ruminants but is generally unreliable in inducing labor in mares. The high doses required, prolonged intervals from treatment to delivery, and potential problems associated with corticosteroid administration (i.e., laminitis) preclude the routine use of glucocorticoids for induction of labor in the mare. In addition, induction of labor with glucocorticoids has resulted in a higher proportion of weak foals, prolonged parturition, dystocia, and poor milk production.

Prostaglandins

Prostaglandins have been used successfully in mares, but the interval to rupture of the chorioallantois (i.e., the mare ‘breaking her water’) and subsequent foaling is not consistent or predictable.

Oxytocin

The most common and reliable technique for induction of labor in mares is the administration of oxytocin. The interval from the onset of oxytocin treatment to the onset of early labor (sweating, discomfort, obvious uterine contractions, etc.) is five to 10 minutes, and delivery of the fetus usually occurs 20 to 45 minutes from the onset of treatment.

Several dosage and administration regimens have been reported for oxytocin. These include:

  • Administration of small oxytocin boluses (2.5 to 20 units) intravenously at 10 to 30 minute intervals.
  • Administration of a larger bolus (40 to 60 units) intramuscularly.
  • Administration of 100 units of oxytocin diluted in one liter of saline through an intravenous catheter at a rate of one Unit/minute or until the fetus is delivered. A disadvantage of this technique is that a person needs to remain in the stall holding the fluid bag next to the horse throughout the procedure.

Comparisons between the various methods of oxytocin induction have indicated that there are no differences in the time interval from initial oxytocin administration to delivery of the foal. Lower doses of oxytocin have been suggested to be more physiologic and result in a quieter foaling.

The oxytocin protocol currently used at Colorado State University for induction of labor in mares is presented in Table 2.

STEPPROCEDURE OR COMMENT
1Administer the initial dose of 5 units (0.25 mls) of oxytocin intravenously.
2Administer a second dose of 10 units (0.5 mls) 15 minutes later.
3Most mares will ‘break their water’ within five to 10 minutes after the second dose of oxytocin and the fetus will generally be delivered within 20 minutes after the second dose.
4If no progress has been made within 15 minutes after the second dose, a vaginal examination of the mare should be performed to determine the position of the fetus within the birth canal; if needed, the outer chorioallantoic membrane may be manually ruptured.
5If the fetus is lined up in the birth canal (i.e. no abnormalities of presentation, position or posture), additional 10 unit doses of oxytocin may be administered at 15-minute intervals.
6Occasionally, it is necessary to apply obstetrical chains or straps and provide traction to facilitate delivery of the foal.
7Once the induction protocol has been initiated the mare is committed, and personnel should remain on site until the foal is delivered.

Mares are closely monitored during the last few weeks of gestation by observing physical signs such as udder enlargement, engorgement of the teats with colostrum, waxing of the teat ends, and relaxation of muscles and supporting structures around the tail head, buttocks, and vulva.

The mare is moved to a large, clean, dry, well-bedded foaling stall five to seven days prior to the expected foaling date. Milk calcium levels are monitored as the mare approaches her foaling date. 

When physical signs of impending foaling are present and milk calcium levels are rising (and preferably above 200 ppm), the mare’s tail is wrapped, and her perineum and mammary gland are washed with soap and water, rinsed, and dried. When all personnel and equipment are assembled, the induction protocol is initiated.

A few minutes after receiving the first dose of oxytocin, most mares go into a natural progression of mild uterine contractions, progressing to moderate uterine contractions, which presumably allows time for the fetus to become oriented appropriately in the birth canal.

Dystocia and other obstetrical complications are very rare using the ‘low dose oxytocin technique.’ In contrast, administration of a large dose of oxytocin may cause a more rapid and more pronounced uterine contraction, and therefore, not allow as much time for proper orientation of the foal or relaxation of the cervix.

The first stage of labor may be abbreviated or absent in some mares, and the second stage of labor may proceed rapidly as a result of the induction procedure. In addition, induced labor may progress faster in mares that have had multiple foals than in mares having their first foal.

Advantages and Disadvantages of Induction of Parturition

Induction of a controlled vaginal delivery may be beneficial when managing the foaling of a mare with a pelvic injury, a history of dystocia, loss of a previous foal due to failure of the outer placental membranes to rupture leading to premature placental separation or ‘red-bag,’ or loss of a previous foal during an unattended foaling.

Induction of labor may also be beneficial when managing a mare with a history of injuring a newborn foal shortly after foaling (i.e., foal rejection). 

Induction of labor can allow for attended daytime foaling and reduce the labor and stress associated with foaling out high-risk mares in the middle of the night. Conversely, induction of labor can be disastrous if guidelines for the selection of mares are not followed.

Unfortunately, if an accurate breeding date is not known, a mare can be induced to foal several weeks prior to her actual due date. Induction of labor in a mare that is beyond her due date, but not exhibiting clinical signs that foaling is near, can result in delivery of a dysmature foal. 

Induction of labor can be associated with dystocia and may lead to a higher incidence of retained placenta. Induced parturition may also be associated with premature placental separation, fetal hypoxia and dysmaturity, and failure of passive transfer.

About the Author

Patrick M. McCue, DVM, PhD, DACT

Dr. Patrick McCue, a distinguished graduate from the University of California, Davis in 1986, is an esteemed veterinarian specializing in equine reproduction. Following his veterinary degree, he completed an internship at the University of Pennsylvania and a residency at UC-Davis, where he also earned a PhD in Comparative Pathology focusing on reproductive endocrinology and ovarian pathology in mares. In 1991, he became a Diplomate of the American College of Theriogenologists. Since joining Colorado State University in 1994, Dr. McCue has served as the Iron Rose Ranch Professor of Equine Theriogenology, leading clinical Stallion and Mare Services at the Equine Reproduction Laboratory. He has authored over 400 scholarly works, including 10 books, and received numerous awards, such as the Norden Distinguished Teacher Award, the American Association of Equine Practitioners Teaching Award, the 2017 Theriogenologist of the Year, a Lifetime Achievement Award from the European Symposium on Equine Reproduction in 2022, and the Frank J. Milne ‘State-of-the-Art’ Lecture Award in 2023.

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