Before labor is induced in a mare, the ability of the fetus to survive extrauterine life must be confirmed. Several physiologic processes occur within the fetus before delivery to ensure that the foal will be viable after birth. The equine fetus is unique in that final maturation occurs only 24 to 48 hours before delivery. Consequently, the equine fetus is at substantially greater risk of dysmaturity/prematurity if delivered at an inappropriate time.
Several indicators have been identified that suggest fetal and maternal “readiness for birth.” Gestational length (>330 days) is often erroneously used by those considering induction of parturition in the mare. The normal gestation period in the mare is highly variable between animals and ranges from 320 to 360 (-340) days. Most mares tend to have a similar gestational length from year to year; thus historic information can be very useful. However day length can affect gestational length so that mares foaling during short days typically have a longer gestation than mares foaling during long days. Therefore all fetuses are not necessarily mature at 330 days from the last breeding. The length of a mare’s gestation should only be used in conjunction with other signs when the decision is being made to induce parturition.
Mammary development and colostrum production in the mare are presently the most reliable indicators of fetal maturity and “readiness for birth.” Colostrum is paramount to the survival of the neonate both as a source of nourishment and immunoglobulins. Furthermore, concentration of mammary secretion electrolytes has been well correlated with fetal maturity in horses. Calcium concentration rises sharply in mammary secretions of most normal mares 24 to 40 hours before foaling. Additionally, sodium concentration typically is much higher than potassium until 3 to 5 days before birth, at which time the sodium to potassium ratio inverts. Changes in mammary secretion electrolytes (calcium, sodium, and potassium) have been compared with neonatal parameters that are indicative of adequate maturity at birth. A rise in mammary secretion calcium greater than 10 mmol/L and inversion of the sodium-potassium ratio are well correlated with fetal maturity in foals.
Precise measurement of mammary secretion electrolyte concentrations requires a flame spectrophotometer or a laboratory chemistry analyzer. With these systems, elevation of mammary secretion calcium to greater than 40 mg/dL and potassium concentration greater than sodium (i.e., potassium >30 mEq/ml and sodium <30 mEq/ml) usually indicates fetal maturity in the normal equine pregnancy. Stall-side tests are available to measure calcium (Ca++) or calcium carbonate (CaC03) concentration. Test kits that measure mammary secretion Ca++ typically use pads on a test strip that change from green to red (Predict-A-Foal, Animal Health Care Products, Vernon, Calif.) or titrate a diluted sample until an indicator dye changes from pink to blue (Titret, CHEMetrics, Calverton, Va.; Sofchek, Environmental Test Systems, Elkhart, Ind.). The dilution kits are somewhat more labor-intensive than the test strip kits. Of the commercially available mammary secretion test kits, the Titret test kit is the most reliable and repeatable test for predicting foaling within 24 hours. In one study, the mammary secretion CaC03 was between 300 and 500 ppm in most mares that foaled within 12 to 18 hours of testing. Mares with mammary secretion CaC03 less than 200 ppm had less than a 1% chance of foaling within 24 hours of testing.
As with other induction criteria, care must be taken when changes in mammary secretion electrolyte concentrations are interpreted. Changes occur most often at night, which is when the majority of mares foal. Therefore samples taken early in the day may not reflect electrolyte changes that occur in the evening or at night shortly before parturition. Mares foaling for the first time may show rapid or no change in electrolyte concentrations before foaling. “Maiden” mares often do not have significant mammary development and colostrum production until immediately before parturition. Conversely, mares with twins or placental pathology may precociously develop a mammary gland, and mammary secretion calcium levels may rise prematurely. Thus although highly reliable for predicting fetal maturity and impending parturition in the normal, multiparous mare, mammary secretion electrolytes may be less useful for maiden mares or mares with abnormal pregnancies.
The importance of cervical dilation before the induction of parturition in the mare has been a point of great controversy. Numerous studies cited in the human medical literature associate poor cervical relaxation with failed induction, prolonged labor, and a high rate of cesarean deliveries. Reports in the veterinary medical literature suggest that inductions may proceed successfully in a mare with a tightly closed, mucous-covered cervix as late as the end of first stage labor. In one study, mares with spontaneously dilated cervixes (determined by digital examination per vagina) before induction delivered their foals more quickly than those mares with a closed cervix. Foals delivered rapidly stood and nursed more quickly and had fewer signs of intrapartum asphyxia (e.g., hypercapnia, maladjustment) than foals that experienced prolonged delivery. Prostaglandin E2 (PGE2) has recently been used to promote cervical relaxation before induction of mares. PGE2 is routinely administered to women to dilate the cervix before induction of labor. PGE2 enhances cervical dilation and eases delivery in treated mares. Foals delivered from PGE2-treated mares suckle more quickly than foals from control mares. Results from these two studies indicate that cervical relaxation before parturition can positively affect neonatal health.
In summary, no one criterion effectively predicts the success of an induced parturition in the mare. Adequate udder development, changes in mammary secretion electrolytes, and cervical softening are all important considerations before induction.