MARE PREPARTUM CHANGES

Progesterone concentrations in the peripheral plasma are less than 1 ng/ml during the last half of gestation; thus, measurement of this hormone is useless for determining deficiencies or predicting the time of parturition.¹ Other progestogens, 20α-hydroxy-5β-pregnan-3-one and 5β-pregnane-3α,20α-diol, decrease during the last 24 hours prior to parturition.^2,³ Prepartum changes in the mare include enlargement of the udder 4 to 6 weeks prior to delivery. This time varies with the individual animal and is somewhat dependent on the number of previous pregnancies, if any. The udder becomes more rounded as it enlarges. Enlargement and edema of the udder and the abdominal wall immediately anterior to the udder are apparent 2 to 3 weeks prior to foaling. Waxing, or the appearance of a sebaceous-like secretion on the teat ends surrounding the sphincter, should occur as the mare approaches parturition. However, waxing has been observed to occur as early as 4 weeks prior to delivery and may not occur in some mares. Waxing generally occurs with regularity in the same animal at approximately the same time prior to parturition. The teats usually fill and distend 2 to 14 days prior to parturition, but this may not be observed. Some mares leak colostrum prior to parturition. This is undesirable because once the colostrum is lost from the udder it is not replaced. Colostrum should be collected and frozen or substitute colostrum should be made available for the foal following delivery.

Relaxation of the tailhead and pelvic ligaments occurs gradually over several weeks before delivery. During this same period the vulva relaxes and the opening elongates. At 1 to 3 weeks prior to foaling, relaxation becomes apparent in the flank area with the mare appearing somewhat “gaunt” in this area. Approximately 7 to 10 days later, the gluteal muscles and the muscles adjacent to the tailhead relax and appear less evident, with the bones in that area more pronounced. The ultrasonographic character of the fluids within the uterus also changes during the last 10 days of gestation, and echogenic particles appear. The vagina and vulva relax and there is an increase in mucus. The vaginal mucosa may appear reddened; this generally occurs within 24 hours of parturition. The cervix becomes completely relaxed and the uterine contractions increase in frequency and strength. Because some mares foal with only a few of these normal prepartum signs, prediction of the exact time of foaling is difficult without further diagnostic tests. Body temperature has been suggested as a possible method to determine the time of parturition before the onset of stage I of labor.^4,⁵ Although it has been demonstrated that changes in body temperature measured at 6-hour intervals with an electronic rectal thermometer were not adequate to predict parturition, the use of telemetry with a computerized system to record core body temperatures at 15-minute intervals could be useful in parturition prediction.^6,⁷

Several commercial aids are available for use in determining approximately when parturition will occur. These include the Sofchek Test strip, the Titrets Calcium Hardness Test Kit, and the Predict-a-Foal Predictor Kit. The Sofchek Test strips indicate there is a 79% probability of parturition within 24 hours when the hardness test strip exceeds 250 ppm. The Titret Calcium Hardness Test strip indicates a 53% probability of parturition within 24 hours when the hardness exceeds 250 ppm. The Predict-a-Foal Predictor Kit indicates a 59% probability of parturition within 24 hours if the color change is a 4 or greater. These tests are more predictive of indicating when the mare will not foal rather than when parturition will occur.

A number of methods commonly are used to monitor the actual parturition. The most frequently used and oldest method is to stay in the barn with the mare and personally observe. This method obviously has several disadvantages including inclement weather and loss of sleep. This system is not 100% effective, and many people have fallen asleep while supposedly observing the foaling or have gone to the house for a few minutes and have missed the birth. Others systems designed to improve the quality of life of the foaling attendant include video cameras, sound monitors, the Foal-alert system, the Birth Alert system, and other systems designed for the mare at parturition. Video cameras have progressed considerably from the days of the poor image quality requiring a great deal of light and hard wiring. The systems of today require minimal lighting and no wires. The cost of these systems also has been greatly reduced, making them affordable to most horse farms. They permit the person monitoring the mare to remain in a more comfortable environment than would otherwise be possible. The author has used several video monitoring systems over the past 22 years and has found them to be worth the cost. Sound monitors including baby monitors and intercoms are of limited value because many horse sounds may “wake” a horse owner and not be related to foaling. However, they do work in combination with other systems such as the monitors, which emit an audible signal at the time of parturition. The Foal-alert system is a monitoring device that includes the placement of a transmitter on the vulvar lips such that at the time of parturition the lips spread with passage of the fetus and the transmitter is activated. The receiver is activated and it produces an audible sound, which can be heard within the barn or transmitted by phone or pager to a distant point. An intercom system can also be used to transfer the alarm sound to a house or sleeping area of the foaling attendant. The Foal-alert system works excellently if any portion of the fetus or fetal membranes filled with fluids reaches the vulvar lips and causes activation of the transmitter. The Birth Alert system is another foaling monitoring system that utilizes a transmitter in the anterior vagina and is activated when the transmitter is expelled from the vagina and the temperature sensor signals the decrease in temperature to the receiver. The receiver transmits an audible signal.

Restlessness increases as the mare approaches parturition. Prior to this increase, mares may demonstrate decreased activity that could last for several weeks. Personality changes are obvious as the time of parturition approaches. This is especially true during the 24 hours before birth. Distractions of any kind should be kept to a minimum during this period. Changes in prepartum myometrial activity have been reported.^8,⁹ Internally, during this period, the mare’s uterus begins more active contractions beginning at the end of the uterus near the ovary and progressing posteriorly toward the cervix.

The fetus is in a variety of presentations until approximately the sixth month of gestation when it aligns itself in a cranial presentation (Fig. 14-1). Presentation may change after this stage, however, and therefore cannot beused reliably to determine the presentation of the fetus until parturition actually begins. The fetus assumes a variety of positions, including dorsosacral, for longer durations as term approaches. As parturition approaches, the head and forelimbs can be palpated in the pelvic canal of the mare and may at times be alongside the cervix with pressure on the vaginal wall.

Fig. 14-1 Fetal presentation and posture 2 to 3 months before birth.

(From Threlfall WR: Threlfall’s equine obstetrics. Powell, OH: Global Reproductive Services, 1996.)

There is no need to wrap the mare’s tail, wash the perineal area, or wash the udder before foaling. These procedures, although recommended by some, mostly serve to disrupt the mare and have no medical basis. However, cleaning the stall and maintaining reasonable cleanliness are especially important before and immediately post partum. This could include disinfecting stalls between mares but is not practical with most farm facilities. For disinfectants to be effective, all organic material must be removed. This is usually only feasible with a high-pressure sprayer and requires that relatively nonporous material be used in the construction of stalls, including the floor. Stalls should be at least 12 ft by 12 ft or larger and be bedded preferably with large quantities of straw. Other types of bedding material may promote bacterial growth or be too dusty for use with neonates or postpartum mares. If Caslick’s procedure has been performed to improve vulvar conformation, the vulva should be opened approximately 14 days prior to the due date or as determined by visual assessment of the mare’s preparation for foaling.

Stage I

Sweating initiated in the shoulder area combined with frequent observations of the flank area by the mare are indicators that stage I of labor is occurring. Kicking, biting of the flank, or other signs of mild colic frequently occur during this period. During this stage, final positioning and posturing of the fetus occur. The front feet and the nose make an excellent wedge to aid in cervical dilatation. This is also assisted by pressure of the fetal fluid on the cervical opening. The increase in pressure is first due to increased uterine activity. As the pressure increases and the cervix starts to dilate, oxytocin is released. Oxytocin increases the uterine contractions and indirectly stimulates the abdominal muscles to contract and further increase the uterine pressure. The beginning of stage I of parturition is difficult to determine because its onset is not marked by any single event or change in the mare and is rather vague and variable. As the normal mare approaches foaling, she may have a decrease in activity the day before the night of foaling. It has been reported that approximately 85% of mares foal between 7:00 PM and 9:00 AM, with most mares foaling between 11:00 PM and 4:00 AM.⁴ Within 2 hours prior to foaling, the mare may become restless and show signs of colic. She may paw the ground and stop eating for short periods of time as an indication of pain. Increased switching or movement of the tail is also usually observed as the mare approaches parturition. Milk may be seen squirting from the teats. This activity is associated with oxytocin release and apparent uterine pain. Signs of pain are not constant but intermittent and correspond with uterine contractions. These periods may last from 1 or 2 minutes to 20 minutes in length. These events may not be observed in older mares, even though the entire process may take several hours. This explains how mares are able to foal during the 30 minutes an observer may be absent. The mare should be observed but left undisturbed unless problems arise. The average range for this period is 1 to 4 hours. Any disturbances may delay the foaling and result in complications. The chorioallantoic membrane ruptures and the allantoic fluid escapes, marking the end of stage I.

The fetal heart rate prior to the onset of parturition is approximately 76 beats per minute, with elevations occurring with increased activity. Heart rate may increase by as much as 40 beats per minute and the accelerations may average one per minute. The activity of the fetus is most pronounced during the period of 5 to 72 hours prepartum. The fetal heart rate during the 24 hours prior to delivery is approximately 62 beats per minute and during stage I and stage II is between 54 and 60 beats per minute. The heart rate of the neonate averages over 100 beats per minute.

Stage II

Signs of the second stage of labor are more apparent. The mare lies down during most of this period; although she may sit up and lie down several times and is generally sweating. When in the recumbent position, all legs may be extended and the head stretched outward away from the body (Fig. 14-2). She may bite her flank area or chew on hay or straw. During stage II of labor, the mare frequently urinates and defecates because of the pressure applied by the contracting uterus and the abdominal wall and the presence of the fetus and fetal fluids. Some mares may roll in an apparent attempt to lessen the pain or position the fetus. The average time for stage II is 20 minutes, calculated from the time of chorioallantoic membrane rupture until delivery of the fetus. It is abnormal for mares to continue this stage of labor longer than 70 minutes, and prolonged second stage indicates problems are present and fetal death is likely. It is generally recommended that mares be given 20 minutes to complete the second stage before the foaling attendant becomes alarmed. Normal progression in the delivery process should occur during this period.

Fig. 14-2 Mare in stage II of parturition with severe abdominal contraction resulting in all limbs extended.

(From Threlfall WR: Threlfall’s equine obstetrics. Powell, OH: Global Reproductive Services, 1996.)

During stage II, the amniotic membrane protrudes from the vulva (Fig. 14-3). The amniotic membrane should be visible at the vulvar lips within 5 to 10 minutes following rupture of the chorioallantoic membrane if the mare has not been disturbed during the delivery process. Once the amniotic membrane is visible, the fetus has entered the birth canal sufficiently to stimulate strong abdominal contractions. As the foal is pushed caudally by abdominal and uterine contractions, the feet are observed through the amniotic membrane. Rupture of the membrane does not usually occur until the fetus is at least midway or completely through the birth canal. Although most mares foal in lateral recumbency, it is possible for parturition to occur while the mare stands. The major concern in this situation is a ruptured umbilical cord close to the abdominal wall and blood loss. If the foal is delivered and the amniotic membrane remains intact as the foal struggles following delivery, the membrane should be incised and removed from the area of the fetal nose and mouth. The normal delivery posture of the fetus is one forelimb extending approximately 4 inches in front of the other and the soles of both feet directed ventrally (Fig. 14-4). This offsetting of the feet results in a slight angling of the shoulders and thereby reduces the width of this area as it passes through the mare’s pelvis. While the foal passes through the vulvar lips surrounded by the amnion, the observer can determine the offset position of the hooves and note that the soles are ventral. If the soles are not directed ventrally, immediate attention is indicated. The head should rest on the forelimbs in the area of the carpal joints. The head and neck are extended. If the nose of the fetus has not appeared by the time the carpal joints are observable immediate intervention is indicated. The vertebrae of the fetus are located near to the vertebrae of the mare; the position is close to a dorsosacral but slightly off toward the ileum. This position permits the fetus to take advantage of the greater pelvic diameter. As the fetus is expelled further, the rear limbs are extended so that the hooves are the last part of the fetus delivered. Any variation from this normal position, such as failure to observe one of the feet or the head and neck, indicates a postural abnormality and a difficult delivery. Because the mare’s delivery is so forceful, she may severely injure herself by pushing the fetus out and tearing her tissue as the fetus is expelled. Also, anything that slows the delivery process is detrimental because the placenta separates rapidly and eliminates the fetal supply of oxygen. This probably accounts for the poor survival rate of foals when the mare was in need of assistance and none was immediately available.

Fig. 14-3 Mare in stage II of parturition with the forefeet protruding covered by amniotic membrane.

(From Threlfall WR: Threlfall’s equine obstetrics. Powell, OH: Global Reproductive Services, 1996.)

Fig. 14-4 Normal presentation, position, and posture. Anterior longitudinal with dorsosacral position and extension of head, neck, and forelimbs.

(From Threlfall WR: Threlfall’s equine obstetrics. Powell, OH: Global Reproductive Services, 1996.)

Stage III

The mare should be permitted to lie quietly in the stall for up to an hour following parturition; however, if she jumps up rapidly following delivery, it is not necessarily a reason to panic. Although it has been suspected, it has not been demonstrated that abdominal wall damage occurs to the fetus when this occurs. The determination of hemorrhage from the umbilicus, although not common, should be made. It was once believed that large volumes of fetal blood could be lost from the placenta and that care should be taken to assure the cord remained attached to the placenta. A more recent scientific study revealed that blood flow through the umbilical vessels ceases by the time the umbilical cord is visible at the vulvar area of the mare.¹⁰ Following delivery of the fetus and after the mare has accepted the foal and is resting, cleaning of the stall and observation of the mare and foal are the two most important managerial tasks. Stage III of parturition begins following expulsion of the foal and ends when expulsion of the placenta is complete. The fetal membranes should be expelled within 3 hours following delivery.

INDUCED PARTURITION

Parturition induction permits observation and professional assistance, if necessary, at the time of foaling. There are primarily two reasons for parturition induction. The first is for medical indications. It is of most value when used for mares that have a history of difficult deliveries or have had injuries or illnesses that could possibly endanger the life of the mare or the foal if not assisted immediately at the time of parturition. Among the clinical indications for induced parturition are ruptures of the prepubic tendon, rupture or weakening of the abdominal wall, following surgical correction of third-degree perineal lacerations or rectovaginal fistulas, previous fracture of the pelvis with narrowing of the birth canal, history of premature placental separation, prolonged gestation, near-term colic, and uterine inertia. Parturition induction also permits opening of Caslick’s vulvoplasty immediately prior to delivery and replacement immediately post partum, thereby reducing contamination of the reproductive tract. Furthermore, if an episiotomy is indicated to eliminate vulvar lacerations, it can be performed when indicated. Induced parturition in mares that have previously produced foals with neonatal isoerythrolysis can be easily managed by removal of the foal immediately following delivery.

The second reason for the induction of parturition is for convenience. Induction of parturition is becoming an acceptable method of permitting the owner, manager, or veterinarian to be present at the time of delivery in normal mares. The value of induced parturition in labor savings is brought about by the elimination of “foal watchers,” people hired to observe mares near term, or by the elimination of frequent observations throughout the night with the possibility of missing the foaling onset. The labor savings plus the guarantee that capable assistants will be present at the time of parturition easily offset the expenses incurred in parturition induction. The convenience factor also applies to certain research projects involving parturition and neonatal physiology and for the educational value of being able to provide a parturition demonstration. Induction has also been utilized to induce mares to deliver where the primary intention was to provide a nurse mare for a valuable foal. There is no association between induction and impaired future reproductive efficiency. Owners must be aware of the problems with dysmature or premature foals if delivery occurs prior to the time the foal is mature.

Some of the more general criteria to consider before induction of parturition include knowledge of the gestation length, which should exceed 320 days. The mare’s previous gestation lengths can be utilized to some extent to determine term but the effect of time of year should be considered. Mares foaling in the months of January, February, and March have longer gestations than mares foaling in April, May, and June. Prepartum changes such as perineal and cervical relaxation have been used as indicators for predicting parturition but may not be accurate enough to predict the time to induce parturition. Although relaxation of the pelvic ligaments can also be used as an indicator of approaching parturition, it is less noticeable than udder changes and more experience is necessary to detect subtle changes. Mares should be introduced into the foaling environment at least 3 to 4 weeks prior to induction if possible. This permits the mare to be exposed to the organisms in the new environment as well as become accustomed to her new surroundings. The mare should be placed in a clean stall bedded with straw in a quiet area of the barn. Rectal examination of the mare at this time usually permits evaluation of fetal presentation, position, and posture as well as viability of the fetus. Because this is an elective delivery and the time of delivery is predetermined, a tail wrap is applied to the mare and the perineal area is washed prior to administration of the induction agent.

Determination of fetal presentation, position, and posture plus a physical examination of the mare must be performed prior to induction. The fetus should be in cranial longitudinal presentation with slight dorsoileal position and the head and neck extended. Although the final delivery position and posture are determined as the mare enters stage I of parturition, the presentation will have already been predetermined. A caudal presentation would indicate the importance of assistance much earlier in the delivery process than a cranial presentation. If the head and feet are not in the proper posture, it is very probable that correction will occur before delivery without assistance. The author does not advise intervention once delivery has been induced unless it is apparent that the progress is slower than normal or is abnormal. Assistance that is too aggressive or premature may result in cervical lacerations or delay parturition.

Contraindications to parturition induction include conditions that indicate that expulsion of the fetus is not possible through a pelvic canal greatly reduced in size. Any abnormal condition affecting the mare prior to induction should be brought to the attention of the owner and a decision made as to whether to proceed. Conditions such as increased body temperature, abnormal vulvar discharges, and abnormal presentation of the fetus should influence the decision to induce because they may affect the outcome. Fetal extraction following cervical dilation without parturition induction may be indicated in certain instances to provide a better probability of fetal survival.

The most common agent used to induce parturition is oxytocin by intravenous or intramuscular administration.¹¹ If the cervix is closed, administration of estradiol cypionate (4 to 6 mg) or diethylstilbestrol may be considered 12 to 24 hours in advance of oxytocin administration to aid in relaxation. Oxytocin also has the capability of inducing cervical relaxation in a mare with a closed or nearly closed cervix. The best results are obtained when it is administered as a 10-unit dose intravenously every 15 to 30 minutes by slow intravenous drip. Four to six repetitions of bolus administration may be required. If dilatation occurs without delivery, the dosage of oxytocin can be increased to 20 units. While determining the degree of cervical dilation, one can also determine the position and posture of the fetus by transrectal palpation because disruption of the mare has already occurred.

The dosage of oxytocin administered varies with the degree of cervical relaxation. If the internal diameter of the cervix is relaxed to at least 2 cm, it has been recommended by several authors to administer 40 to 60 units oxytocin as an intravenous bolus. Delivery of the fetus should occur within 90 minutes. In a pluriparous mare, the sequence may be more rapid, with stage II completed within 30 minutes after oxytocin administration. Many veterinarians have traditionally administered oxytocin intramuscularly.¹² Much of the recent literature has reiterated the observation that the mare’s response (both in speed and violence) is proportional to the dose of oxytocin given. The higher the dose of oxytocin the faster the delivery of the fetus. Oxytocin has been reportedly used to induce parturition in six mares with all foals normal and healthy within approximately 34 minutes. None of the mares had a retained placenta and all lactated normally. The dose of oxytocin ranged from 40 to 60 units. The author prefers to administer 20 units of oxytocin intramuscularly to mares with a minimum of 2 cm cervical dilation and has not had any of the reported problems described later in this chapter.

In mares with at least 2 cm cervical dilation and other signs of preparedness for delivery, the administration of 20 units of oxytocin is followed in approximately 5 to 10 minutes with restlessness and slight colicky pains, which include tail movements and looking at the flank. Within 20 minutes, the mare will be walking, frequent defecation will be occurring, the tail will be held partially up for long periods of time, frequent urinations will occur, she may repeatedly move from the standing position to the recumbent position, and sweat will appear in the area of the shoulder. The movements will become more pronounced and the sweating will extend to other areas of the body. Rupture of the chorioallantoic membrane will occur approximately 30 minutes following oxytocin administration. Delivery of the fetus through the vulva will commence in approximately 30 to 60 minutes following administration and will take approximately 30 to 40 minutes. The fetal membranes should be passed within 3 hours or the mare should receive additional oxytocin as a treatment for the retained fetal membranes.

Reportedly, oxytocin is dangerous when used to induce parturition because it has the capability to induce delivery whether the fetus is mature or not. The response of the mare is to release larger quantities of prostaglandin than occurs during a natural parturition and is therefore a pharmacologic not a physiologic induction. Fetal membrane retention occurred as an undesirable effect in two of three mares when oxytocin was given by slow intravenous drip for longer than 3 hours. Four of five mares induced with oxytocin produced foals that were weaker than normal, and one had to be “destroyed.” Cases of malposition have been reported following induction with oxytocin. It is recommended to examine the fetus 20 minutes following oxytocin administration to determine the fetal position and to correct any malposture or malposition. The author believes that all these side effects could and do occur in natural deliveries and that some of the effects may be related to dose or route of administration.

Prostaglandin F₂ alpha (PGF_2α) reportedly failed to induce parturition in the mare.¹³ PGF_2α can also cause very strong myometrial contractions and may decrease foal survival and increase fetal weakness and risk of death due to early placental separation.^13,¹⁴

Fluprostenol is capable of inducing parturition in mares. Fluprostenol causes less myometrial stimulation than PGF_2α and, therefore, has been successfully used for induction. Mares with a closed cervix can be induced with fluprostenol at a dose of 2.2 μg/kg. The time to delivery should be approximately 4 hours. Fluprostenol was administered to 17 ponies, horses, and donkeys to induce parturition. The prostaglandin F metabolites (PGFM) increased after injection in mares that delivered within 90 minutes following injection and peaked during the maximal expulsive efforts of parturition. If parturition required longer than 90 minutes, the PGFM increased at various times following injection and peaked before the maximal expulsive efforts.

Thirty-three mares were induced to foal with prostalene or fenprostalene based on presence of prefoaling mammary secretion. Twenty-three mares served as control subjects and were monitored but not induced. All mares given fenprostalene delivered within 3.9 hours following injection without complication. Seventy-five percent of mares receiving prostalene delivered within 3.7 hours and the remainder delivered within 30 to 56 hours following injection without difficulty. Tests were performed to determine the content of calcium carbonate or the total “hardness” of the colostral secretion. Mares were monitored beginning 10 days prior to the expected foaling date and the secretion checked daily for 3 days and then twice daily until parturition.

Combinations of fenprostalene followed by oxytocin in 5-unit doses at 20-minute intervals has been recommended as a means to reduce the approximate 4-hour time interval following the prostaglandin analogue administration until delivery. Oxytocin was administered 1 hour following the initial injection and the delivery time was more predictable.¹²

Glucocorticoids have reportedly failed to induce parturition in the mare. Dexamethasone, 100 mg at 24-hour intervals until parturition occurs, has been utilized but with less reliability than oxytocin. The average induction time of 4 ± 1.6 days was considered to be undesirable because qualified assistance probably would not be present when required. Because suppression of the immune response is a disadvantage of corticosteroid therapy, it is thought that the use of this substance to induce parturition may be undesirable.¹³ Combinations of PGF_2α and flumethasone, a synthetic glucocorticoid, have been reported to successfully induce parturition.