Erin E. Runcan, Margo L. Macpherson, Dale L. Paccamonti
Induction of Parturition
Erin E. Runcan, Margo L. Macpherson, Dale L. Paccamonti
Parturition in the mare is an efficient, well-coordinated event. The process begins with stage I, which lasts 1 to 4 hours and is defined as the period in which initial uterine contractions are leading to cervical dilation and fetal positioning. During this time, the mare appears mostly unaffected and continues to eat and move about normally. Stage II, marked by rupture of the chorioallantois, usually is completed in less than 30 minutes. During this stage, the mare becomes inappetent, restless, and sweaty because of active uterine contraction; these stage II contractions result in delivery of the foal. Stage III of labor, which lasts 1 to 3 hours, terminates with expulsion of the fetal membranes. The highly orchestrated events in equine parturition are crucial to the well-being of both the dam and the foal. Intervention during the parturient process, even when medically necessary, can significantly affect maternal and fetal health, but in some instances it is best for trained personnel to be present during foaling. Physical maladies, such as hydropic conditions, ruptured prepubic tendon, and body wall defects may result in poor abdominal press intensity during delivery, which can lead to dystocia in the mare, hypoxia in the foal, or both. Inducing parturition can allow for prompt intervention, and can be life saving for both mare and foal. The risks and benefits of this procedure should be carefully considered so that all involved parties are prepared for the potential outcomes. Reviewing the criteria of a mare’s potential readiness to deliver is an important first step to aid in the decision-making process before inducing parturition in the mare.
Criteria for Induction of Parturition
A prerequisite to inducing labor in a mare is determining whether the fetus is capable of surviving extrauterine life. Several physiologic processes take place in the fetus before delivery, ensuring that it will survive after birth. The normal fetus must have appropriate energy reserves, functional lungs and gut, and the ability to suckle, swallow, and maintain body temperature after delivery. In most domestic species, fetal maturation is associated with a prenatal increase in adrenocortical activity a few weeks before birth. The equine fetus is unique in that there is little adrenocortical activity until 24 to 48 hours before birth. Final maturation of the equine fetus occurs during this period. Consequently, the equine fetus is at substantially greater risk for dysmaturity or prematurity if delivered at an inappropriate time.
Predicting the best time to induce parturition in mares remains an inaccurate science. The average gestational length of the mare, mammary development, mammary secretion electrolyte concentrations, and cervical softening are all factors that should be reviewed before inducing parturition. These factors should be considered together when making a decision because no single criterion alone is effective for accurately predicting fetal readiness for delivery.
Early studies suggested that mares with a minimum gestational length of 330 days would consistently have mature foals. However, because normal gestational length ranges from 320 to 362 days in light horse mares, not all fetuses are mature at 330 days from the last breeding date. Gestational length can be useful, however, if the typical gestational length in a given mare is known because most mares have a similar gestational length from year to year. However, factors such as breed and day length can also affect gestational length. Pony mares can have shorter gestations than light or heavy horse breeds. Mares foaling during short days typically have a longer gestation, whereas mares foaling during long days have a shorter gestation. Given these points, gestational length is an insensitive parameter for determining a mare’s readiness to give birth. This endpoint should only be used in conjunction with other signs when deciding to induce parturition.
Mammary Development and Secretions
Mammary development and colostrum production in the mare are considered to be the most reliable indicators of imminent parturition and fetal maturity. Calcium concentration undergoes a rapid prepartum rise in mammary secretions in the days before foaling, and the relative concentrations of sodium and potassium invert (with K+ concentration becoming higher than Na+ concentration). Analysis of a sample of mammary secretions and detection of calcium concentration greater than 40 mg/dL and K+ concentration greater than Na+ generally indicate fetal maturity in the normal equine pregnancy. However, changes in mammary secretion electrolytes most commonly occur at night, coincident with the period when most mares foal. Laboratory evaluation of mammary secretions is impractical at night, and most owners and practitioners rely on stallside tests to measure calcium (Ca2+) concentration in mammary secretions. Stallside test kits are modified water hardness tests that often measure both divalent cations, magnesium and calcium, in milk samples. The peak concentration of magnesium is reached earlier than that of calcium, and magnesium often declines at the time of parturition. A stallside test that measures magnesium might prematurely indicate that the fetus is “ready” for birth, but the calcium concentration in the secretions might still be low. For this reason, it is critical that only calcium be measured and that either a laboratory assay or a water hardness kit that measures only calcium is used. Stallside tests are best reserved for predicting normal foaling or for use by personnel who are experienced in interpreting values over several days of measurement.
The mare’s parity and placental health can complicate interpretation of mammary secretion electrolyte values. Primiparous or “maiden” mares often have slow mammary gland development, and mammary secretion electrolytes may not change until immediately before parturition. Mares with placental pathology also pose a challenge because they often have precocious mammary gland development with a concurrent rise in calcium that occurs before the foal is ready for birth.