Induction of parturition and abortion (Proceedings)


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.

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).

Methods of induction

Oxytocin: (5 - 100 IU) IV or IM Delivery in 1 hour, With the ultra low dose 5IU, if the mare is not ready to foal she will not respond. The 5IU dose may be repeated daily until the mare foals. At the 20 IU dose of oxytocin mares will respond and should foal in 1 hour. A second injection at 20 - 30 minutes may be needed in some mares. The stage 2 or water breaking should occur around 20-25 minutes after administration of oxytocin. Recommend checking the cervix every 10 minutes to monitor dilation and to check for placental separation. Higher dosages are associated with more complications.

Prostaglandin PGF 2 – α

10 mg IM results in delivery in 4 hours or more. If the mare fails to deliver it is not advisable to give a second dose, as it means the mare was not ready to foal. Use of this hormone may help a mare lactate if the induction is prior to term, but new protocols to induce lactation in mares, means this is less likely to be performed for this purpose. Mares will sweat, cramp and have colic like signs associated with the injection of prostaglandin..


100 mg SID for 4 days, mare usually foal 4 days after the last shot, you may start as early as day 316 of gestation, but colostrum is suboptimal in these cases. Dexamethasone is used to mature foals in cases where it appears the mare will not survive to term.

Intrafetal injection of ACTH or betamethasone (considered experimental)

Prior to induction of parturition a prefoaling examination should be performed.

The prefoaling examination should include: making sure the mare does not have a Caslick in place, a rectal / ultrasound, and vaginal examination. Regarding the rectal and ultrasound examination mare owners usually have the mare's pregnancy examination performed at 11 - 35 days post breeding. These examinations are intended to confirm pregnancy, identify / eliminate twins, and to detect early embryonic loss. These are often the last transrectal evaluations a mare has prior to foaling, unless a mare develops other clinical signs such as vaginal discharge or premature udder development. Ginther and coworkers reported that after about the 7th month of pregnancy the fetus is physically too large to change its longitudinal orientation in the uterus, and therefore the fetus is positioned in either anterior, posterior, or rarely in transverse presentation from around the 7th month to parturition.

Posterior presentation of the fetal foal during parturition in the mare is associated with higher fetal and maternal losses. The preinduction ultrasound evaluation will allow a veterinarian to determine the orientation of the fetus in the mare. It has been reported that around 1% of foals are born in posterior presentation. Therefore close to 99% of the time a fetal foal is situated in anterior presentation in a dorsoabdominal position (with its back along the mares abdominal wall), and its forelimbs flexed. By using ultrasound the presentation of the fetus may be confirmed through the identification of the fetal head and carpal joints. The fetal limbs are readily assessable as is the fetal head through a transrectal approach. The cranium casts a characteristic arch shaped shadow and the fetal orbits ares easy to identify. The length plus width of the orbit may be measured to help determine fetal age. The fetal forelimbs are usually flexed at the carpus. Therefore the carpal joints may be scanned and the 3 separate joint spaces (carpometacarpal, intercarpal, and radialcarpal) may be identified. The growth plate of the third metacarpal bone may also be identified. The growth plate should be smooth and not irregular. If the head and orbit are visualized there should be no doubt about the presentation of the foal, if it is not identified then we encourage a re-evaluation at another time, in other words a non-diagnostic scan was obtained. With practice a posterior presentation of a foal may be diagnosed by the bony structures and joint spaces: tarsometatarsal, distal intertarsal and tarsocrural joint, and the calcaneous and trochlea of the talus. If the mare foals before the second evaluation is performed we encourage the foaling attendant to evaluate the mare after the rupture of the chorioallantois (water breaks) for the presence of 2 feet and a nose.

The combined thickness of the uterus and placenta (CTUP) is measured using transrectal sonography. It has been reported that mares may have increased CTUP without concomitant physical signs. An increase in the CTUP has been reported to be associated with ascending placentitis. Placentitis is a common cause of pregnancy loss, premature foaling, and premature placental separation. Ascending cervical infections usually result in increased CTUP adjacent to the cervix. The area for measurement of the CTUP is cranial and lateral to the cervix on the ventral portion of the uterus. The cervical branch of the middle uterine artery is used as a marker of the correct location to perform the CTUP measurement. Multiple measurements are taken on the ventral surface of the placenta and the mean CTUP is recorded. Guidelines for CTUP in late pregnancy are <10mm 301 - 330, <12 mm after 330 days. Higher CTUP values indicate that the placental thickness has exceeded the 95% confidence interval, and may suggest placentitis is present. Therefore other features such as a CBC, progesterone, echogenicity of the fetal fluids, and fetal heart rate should be evaluated to confirm the clinical findings.

The vaginal examination must be performed in a very careful sanitary manner. The mare's tail should be wrapped the perineum washed, and a clean speculum or a rectal sleeve and surgical glove lubricated with sterile lubricant used to determine if the cervix is soft. If the cervix is tightly closed it may be advisable to wait with the induction. No discharge or a scant slightly clear mucus and blood tinged vaginal discharge are usually noted in mares.

Typical induction

Check for a Caslick and open the Caslick if necessary. The manure is emptied from the rectum. The presentation of the fetus is determined. The mare's tail is wrapped and udder and perineum cleansed. A low dose of oxytocin is administered. The progression of the mare's cervical dilation is monitored every 10 minutes by a careful clean vaginal exam. It is imperative not to confuse the cervix bulging into the vagina with premature placental separation. Typically the chorioallantoic membrane ruptures around 20 minutes after oxytocin is administered. A large volume of allantoic fluid, which is urine - like fluid, is usually released. The fluid will continue to come from the mare as she moves unlike uring. If there is movement present of the vulva without the membranes having ruptured an immediate vaginal should be performed. The movement may be the foal's limbs enclosed in the separated chorioallantois that has separating and is bulging into the vagina. (see below). After the chorioallantoic membranes rupture the amnion is usually noted at the vulva from 5 - 30 minutes later. If there is no progression after 30 minutes a vaginal examination should be performed. The first membrane seen normally coming from the vulva is the amnion which is pale, bluish, and translucent. The foal's legs are usually clearly seen inside the amniotic membrane generally spaced about 10 cm apart. The foal's legs in the amnion may be pushed in and out of the vulva as the mare gets up and down. As the contractions continue and the foal's head engages the mare will usually lie down and use her abdominal muscles to help push out the foal. This portion takes less than 5 minutes. The foals hind limbs usually remain within the mare's vagina, the amniotic membrane may need to be removed from the foal's face. The mare should pass her placenta within 30 minutes to 4 hours from birth, therefore intervention (oxytocin administration) should start at 4 hours to encourage passage of the palcenta.

Premature separation of the chorioallantois

The chorioallantois is red and velvet like. It normally is attached to the uterine wall and supplies the foal with oxygen and nutrients during birth. The chorioallantois should rupture at the cervix inside the mare and should not bulge far into the vagina or out the back of the mare. It is abnormal to have this membrane separate from the uterus so much that it bulges far into the vagina. In order to intervene you will need to first identify the cervix, which may be bulging into the vagina as the uterus contracts. You do not want to cut into the cervix, so be careful and do not confuse the cervix with the placenta. If the placenta is bulging through the cervix far into the vagina you will be able to find the ring of the cervix, and you see the velvet like surface and the cervical star as you part the lips of the vulva. Cut the chorioallantois at the cervical star in the vagina with a scalpel blade and assist delivery of the foal. Foals born after premature placental separation tend to be slow starters and usually have difficulty standing or have poor suck reflexes. They may develop cerebral edema in the next 24 hours and become "barker foals" (neonatal maladjustment syndrome).

The most common interventions during induction are: manipulations to correct premature placental separation, manual traction, and mutation. Occasional repositioning of the foal by mutation is required during an induction. Weak foals may not turn and will be found in the dorsoabdominal position (upside down for birth). Occasionally foals become poll locked and don't get their nose up into the pelvis. Foals in posterior presentation usually require assistance.

More on indications for induction of parturition

Labour may need to be induced to save the life of the mare before the criteria for safe induction are met. Physical compromise of the mare from pregnancy sometimes occurs, and may be related to the effects of the combined excess weight of the fetus, fluids and placenta. Therefore twins, hydropic conditions (hydramnios, hydrallantois), may lead to ventrolateral abdominal muscle disease, or in hydrops the mare may experience respiratory distress [10]. Not all cases result in extreme clinical signs, and most mares carrying twins have late spontaneous abortions. Chronic colic and laminitis may compromise the mare. In these severe cases of physical compromise where the mare's condition is deteriorating, such as in laminitis, chronic colic, or ventrolateral abdominal muscle disease consideration should be given to induction of abortion, induced preterm delivery, terminal cesarian section, or euthanasia [10, 11]. Retained placenta and other sequelae are possible. In specific circumstances such as excessive weight on the mare's ventral abdomen, it may lead to muscular compromise.

Ventrolateral Abdominal Muscle Disease, Prepubic Tendon rupture

The etiopathogenesis of this condition is not usually known although excessive weight on the ventral abdomen is associated with the condition. The majority of these cases are mares carrying single foals and are idiopathic [11]. The mares with ventrolateral abdominal muscle disease may progress to rupture of the prepubic tendon, and the fetus may die. In ventrolateral abdominal muscle disease, the internal abdominal oblique is most commonly affected. Presentation includes excessive, hot ventral edema, and reluctance to move. Physiological edema of pregnancy is not hot and is not painful to the touch. The skin may be discoloured underneath the hair coat. The history often includes that the mare was kicked, or slipped on the ice. However in the majority of cases the mare was not observed to have been kicked or observed to have fallen it was just assumed by the owner. The mares with this condition will be reluctant to move and are in pain. Mares from mid to late pregnancy are affected. Ultrasound examination of the inflamed area will show that the underlying muscles shows have massive swelling and fibre disruption. The muscles of the abdomen should not be more than 1 cm thick during pregnancy. The enzymes AST (myonecrosis) and CPK (muscle damage) will be elevated late in the process. The wait and see approach is dangerous. If waiting overnight the mare should be treated with NSAID's and corseted. If a recheck at 24 hours shows that the ultrasound condition of the muscles indicates extension of the problem, and the enzyme levels are the same or increased this suggests ongoing damage and is an indication to terminate the pregnancy. Bloody mammary secretion indicates that the rectus abdominus is involved. The bloody mammary secretion is considered pathognomonic for the condition and indicates that the rectus abdominus is involved.

Udder displacement indicates that the prepubic tendon has ruptured. Most mares should be induced asap to stop the progression. The muscles are too damaged to attempt any repair at this point, so surgery to repair the damage is not an option. The muscles mare be too compromised to close after a cesarian section. Terminal cesarian sections are sometimes performed to save the foal. Some affected mares will herniate during spontaneous or induced parturition as a consequence of the muscle damage. Extensive damage usually results in rupture of the prepubic tendon, so abdominal contents are then adjacent to the skin. These mares are in a tremendous amount of pain. They develop a profound ileus, colic, and usually die.

Post herniation the surviving mares are treated with hydrotherapy, corsets for support and NSAID's. The mares that survive may have a mesh repair of the hernia for cosmetic and safety reasons 6 – 8 weeks post – foaling after the muscles have had time to heal. A mare may successfully foal after one of these episodes but is has been suggested that they are more likely to repeat the problem in the future, and some valuable mares are used as embryo donors.

Indications for abortion

The common cited indications for induction of abortion in the mare include: mismating, twin pregnancy, maternal compromise from pregnancy, fetal anomaly, or for the purpose of being a nurse mare.

Termination of pregnancy due to mismating in the mare is easily treated with prostaglandin administration to induce luteolysis from day 5 - 35 days when the mare is still dependent on luteal progesterone. Occasionally a twin pregnancy is missed in due to a lack of early examination, and prostaglandin is an affordable and effective means of terminating pregnancy. Other therapeutic options for early pregnancy include transrectal crushing of the embryonic vesicle (up to 40 days), allantocentesis (25 – 60 days), and after 70 days transrectal fetal craniocervical dislocation, or transabdominal fetal cardiothoracic puncture may also be explored [6-8]. These techniques require both specialized equipment, or training. New protocols for the induction of lactation in mares have diminished the need to abort a mare with prostaglandin to allow her to a nurse mare for another more valuable foal [9]. Fetal abnormalities have been described including fetal malformations and abembryonic vesicles. Abnormal embryonic development is often results in spontaneous pregnancy loss or abortion. Induction of abortion may be indicated in some cases to allow the mare to be rebred earlier.

Protocols for the induction of abortion

There are a few reports in the literature comparing techniques to induce abortion. Methods to terminate pregnancy includes:

Prostaglandin (PGF2α) 5 mg SQ / or cloprostenol 125F2α μg/SQ once from 5 - 35 days of pregnancy

After day 35 – daily to twice administration 10 mg PGF2α IM / or closprostenol 250μg/IM to effect. Mares will frequently show transient side effects within 20 minutes of administering PGF2α or cloprostenol such as: profuse sweating, abdominal discomfort / colic-like signs, Flehman, ataxia, and increased defecation [14]. Additional mild colic-like episodes of discomfort may be noted in between the administrations of prostaglandin before abortion occurs. If abortion has not occurred by the third day of treatment, 30 minutes after the administration of PGF2α, 20 IU oxytocin may be administered.

     • Intrauterine infusions of 1L of 0.5% povidone iodine, 100 mls procaine penicillin G, or 100mls to 2L of hypertonic saline

     • Placental membrane disruption - disruption of the fetal membranes such as with a biopsy forcep, or transcervical passage of a pipette

     • Manual dilation extraction. Deposit Prostaglandin E (200 μg intracervically) 2 hours prior to the induction of abortion, or manual extraction.

With any of the methods the fetus may be delivered alive or with the fetal membranes intact or ruptured, necessitating euthanasia; hence sensitivity to owner concerns about animal welfare are important in this regard [16].

Complications related to induction of abortion

Early induction of abortion has few side effects, later in pregnancy the complications associated with foaling may occur such as retention of fetal membranes, dystocia, endometritis, and endotoxemia. The fetal membranes should be evaluated for completeness after passage. The size of the fetus after 8 months of pregnancy may lead to a higher likelihood of dystocia, therefore it may be advisable to let the mare go to term. General management after abortion includes monitoring the mare's tpr, appetite and demeanour. Vaginal discharge, fever, inappetence or depression are indications for further evaluation of the mare including a detailed examination of the reproductive tract [12].


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Honnas, C.M., et al., Hydramnios causing uterine rupture in a mare Journal of the American Veterinary Medical Association, 1988. 193(3): p. 334-336.

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Daels, P.F., et al., Endogenous prostaglandin secretion during cloprostenol-induced abortion in mares. Animal Reproduction Science, 1995. 40(4): p. 305-321.

Volkmann, D.H. and K.G.M.d. Cramer. Prostaglandin E2 as an adjunct to the induction of abortion in mares. in Equine Reproduction V: Proceedings of the Fifth International Symposium on Equine Reproduction. 1991: Journals of Reproduction and Fertility Ltd.

Varner, D.D., et al. Effect of equine abortifacients on fetal viability and post-abortion reproductive performance. in Proceedings, 11th International congress on Animal Reproduction and Artificial Insemination, Dublin, Ireland June 26th-30th, 1988. Volume 2. 1988.

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