Induction of parturition and abortion (Proceedings)
Induction of parturition
Veterinarians are often asked to induce parturition or they may recommend induction based on the mare's foaling history or the presence of medical conditions that threaten her health and well being. The need to induce a mare, and the chances of foal survival should be based on objective measures. Failure to have a successful outcome may lead to legal action. It is critical that we act in the best interests of our equine patients, which may not be aligned with the perceptions or desires the owner has. The indications, criteria, and methods for the induction of parturition and abortion in the mare will be reviewed. The literature on pregnancy in most species describes cases of maternal and fetal mortality and morbity, associated with labour and delivery. Pregnancy is a statistical risky process for the mother. A mare owner who breeds their mare thus has started a process that if it leads to conception and pregnancy carries inherent risk.
Biologically the mare's pregnancy is dependent on luteal progesterone secretion from the primary corpus luteum through at least the first 35 days of gestation. The formation of endometrial cups, which secrete equine chorionic gonadotropin (eCG) from 35 days to around 120 days of pregnancy, follows and lead to the formation of supplementary corpora lutea. Around 70 days of gestation the mare begins to secrete placental progestagen which gradually replaces luteal progesterone for pregnancy maintenance [1, 2]. The placental progestagens maintain pregnancy in the mare. The mare has high levels of testosterone and estrogen during pregnancy as a result of the production of androgen precursors from the fetal gonads. High estrogen levels mean that mares have abundant oxytocin receptors during pregnancy, and are thus more responsive to exogenous oxytocin than other species. Just prior to parturition the fetal adrenal activates and fetal cortisol and progesterone increases. Shifting steroid ratios at the end of pregnancy with more biologically potent estrogen being secreted results in the onset of labour like contractions, and parturition. Pregnancy loss or termination during time of endometrial cup secretion does not intercept the secretion of of eCG, rather the luteinization of large follicles continues, (supplementary corpora lutea formation), and mares therefore fail to return to estrus [3-5] under the cup tissue regresses. This paper will review the indications, criteria, and methods for the induction of parturition and abortion in the mare.The Indications for induction of parturition are many and include the following: previous depressed or stillborn foal, premature placental separation - red bag delivery, previous severe perineal lacerations, vicious behaviour towards foals, severe ventrolateral abdominal muscle disease, intractable colic, severe laminitis, pelvic injury, posterior presentation, uterine atony, excessively prolonged gestation, neonatal isoerythrolysis, fetal distress, placentitis, thickened placenta, research, and teaching. The most common reason for requesting induction of parturition in the mare is owner convenience, which is not on our list of indications.
The mare should meet the criteria for safe induction, and these are very important for the health of the mare and her foal and for legal reasons. The mare must be at least 330 days of gestation. She must have colostrum in the udder (specific gravity >1.030), relaxed sacrosciatic ligaments, and a relaxed cervix (ideal). Very important also is a determination of calcium and potentially other electrolytes such as sodium and potassium in the mare's prefoaling mammary secretion. There are many kits and test strips available that measure the calcium ion, or calcium and magnesium concentration in the prefoaling mammary secretion. Most tests describe a calcium level of 40 mg/dl or >200 mmol/l, according to the units of the test, to indicate readiness to foal. Many mare owners or managers use these tests so they know when to closely observe the mare for foaling. If a lay person is performing the test it is recommended that a qualified lab or practitioner repeat the test to confirm the result. There are opportunities for errors in the assessment of the prefoaling mammary secretion. For example the test kits and strips use diluted prefoaling mammary secretion of one part milk to five parts deionized or distilled water. If the secretion is not diluted correctly or diluted with non-deionized water, the test will not be accurate or reliable. Once the calcium has risen above the threshold, about 70% of mares foal within 72 hours. Changes in the sodium and potassium ion levels follow the rise in calcium. The sodium level falls and the potassium rises. After the inversion, (where potassium levels become higher than sodium) of these 2 ions the mares will generally foal within the next 24 hours.
If it is necessary to induce a mare an informed consent disclaimer is recommended. The consent should state that the owner was apprised of the risks associated with the induction to the mare and the foal. Parturition is not risk free. In any large population of mares that foal, there will be some maternal and fetal losses. The decision to breed the mare in the first place rests with the owner. Birth is a risky business. With careful case selection your presence as a professional and the process of induction should minimizes the danger to the mare and foal. Induction out of the viable window could result in litigation, and a non-viable foal or foal that requires intensive care may result. Other consequences include that the mare may not lactate or may retain her placenta, the foal may be non-viable or require intensive care. There are occasions when the mare has medical conditions that compel the veterinary practitioner to induce the mare before all of the criteria are met. Labour may need to be induced to save the life of the mare. Conditions such as: Impending prepubic tendon rupture - ventrolateral abdominal muscle disease is the main indication, or hydrops of the fetal membranes. Retained placenta is a possible sequelae of preterm induction of labor. High risk mares either have parturition attended or may need to be induced under certain conditions such as placentitis, or fetal compromise (fetal biophysical profile).