PARTURITION

Cheryl Lopate, Michelle LeBlanc, Reg Pascoe and Derek Knottenbelt

The duration of pregnancy in the mare is normally said to be between 335 and 342 days.¹ However, there is a naturally wide range of normal gestational length (320–400 days).^2,³ Mares that foal in the early spring (February and March in the northern hemisphere breeding season) tend to have longer gestation lengths than those foaling in late spring and summer. It is believed that there is a period of embryonic diapause (a period of time during which the developing embryo stops growing and simply remains the same size), which may vary in length from a few days to weeks, depending on the individual mare. This may in part account for the natural, well-recognized variability in gestation length in mares.

As a rule, the best place for the fetus is in the mare’s uterus until both the mare and the fetus are ready for delivery. Decreased placental function resulting in slower development of the fetus may, however, account for some cases of prolonged gestation, such as foals that are delivered after 365 days of gestation yet are of normal size and similar maturity to their peers who foaled over 30 days earlier.

PHYSIOLOGICAL CHANGES THAT OCCUR BEFORE FOALING ⁴

Prior to the delivery of the normal fetus, it must be fully mature and capable of surviving outside the confines of the uterus. All body systems must finalize their maturation in preparation for birth. Increased fetal cortisol secretion in late pregnancy results in maturation of the respiratory and digestive systems. Fetal activity increases significantly in the last few days of the pregnancy. The density of the fetal fluids increases in the last week or two of pregnancy as well.

Signs of impending parturition include:

• Relaxation of the pelvic ligaments and perineum.

• Mammary gland development (Fig. 8.1).

Fig. 8.1 Mammary gland development in a pregnant mare.

• Waxing of the teats (see Fig. 7.37).

• Changes in mammary gland secretions.

Relaxation of the pelvic ligaments and perineum

The ligaments along the tail-head relax and become very soft. Relaxation of pelvic ligaments becomes noticeable from about 2 weeks prepartum and the muscles of the croup and tail-head become progressively more slack, pliable and soft as parturition approaches; this is usually obvious during the day or two before foaling.

Manipulations of this area will result in a wave-like motion of these muscles. The vulva and anus will soften and relax. Lengthening and edema of the vulva may first be noted up to several weeks before foaling and will increase as foaling approaches.

Mammary development and secretions

Udder development begins approximately 1 month prior to parturition, with the most noticeable changes occurring during the last 2 weeks. It occurs earlier in younger mares and later in old, multiparous mares. Final mammary enlargement occurs within 24–48 hours of foaling.

The nature and content of the mammary secretions change slowly over the last month of pregnancy. Initially, the secretions are yellow and watery (serous). As foaling approaches, the secretions thicken and become tenacious, rather like weak honey. This thickening, along with increased mammary engorgement, is indicative of colostrum production. The secretions then change to a thick milky-yellow secretion typical of good-quality colostrum. Milk production may be low with this type of secretion, but as the colostrum is consumed and the secretions become more milk-like the volume will increase significantly.

• Premature lactation is often a sign of impending abortion; thus mares that produce and possibly leak milk at an early stage in the pregnancy must be carefully assessed.

• A mare that shows any premature lactation should be classified as ‘high-risk’ (see p. 252) and full fetal viability testing should be undertaken.

A wax-like secretion may be seen on the teats (see Fig. 7.37). Wax is part of the initial tenacious colostrum fluid produced by the mammary gland. Waxing typically occurs up to 72 hours prior to foaling, although in some mares it may occur up to 2 weeks prior to parturition. There may be only a drop or two of wax or up to several inches of secretions. Some mares may leak colostrum prior to foaling. If the mare is streaming milk for an extended period of time (hours at a time, or several times a day), she should be milked out and the colostrum frozen and saved for the foal, rather than taking the chance that all the colostrum might be lost prior to foaling.

Concentrations of electrolytes in the milk will change in the days before foaling (Fig. 8.2).^5,⁶ These are useful indicators of fetal maturity and are therefore helpful to some extent in determining foaling dates and in assessing the appropriateness of induced parturition (see p. 279). Milk electrolyte levels have been shown to be potential predictors of foaling (Table 8.1), or at least the times when the mare is unlikely to foal. However, they should not necessarily be relied upon to provide a definite indication of impending parturition.

Table 8.1

Alterations in the electrolyte content of mammary secretions in the immediate prepartum period

Fig. 8.2 The approximate changes that occur in electrolyte concentrations in mammary secretions in the normal pregnant mare in the days leading up to delivery (day 0, arrow).

• Milk electrolytes, and calcium in particular, can be measured colorimetrically or by flame photometer in an aqueous aliquot of the centrifuged specimen of mammary secretion.

• Commercial kits are available to measure calcium levels using a rapid strip dry chemistry test (i.e. water hardness testing kits).⁷ These tests are convenient and helpful but were designed to detect calcium concentrations in water (Sofchek, Environmental Test Systems, Elkhart, IN, USA; Titrets Calcium Hardness Test Kit, CHEMetrics Inc., Calverton, VA, USA). The tests need to be calibrated as they are usually working at the limits of their range.

• Kits designed specifically for horses are available commercially (Predict-a-Foal Mare Foaling Predictor Kit, Animal Healthcare Products, Vernon, CA, USA).

• The specific equine test strip has been shown to be easier to interpret but had wider variations than the tests using water hardness values.⁸

• An increase in calcium to greater than 10 mmol/l, a decrease in sodium to less than 35 mmol/ml and an increase in potassium to greater than 80 mmol/ml are all indicators of fetal maturity and impending parturition.

ENDOCRINOLOGY OF PARTURITION

Late-term pregnancy in the mare has a unique hormonal environment (Fig. 8.3).^9–11

Fig. 8.3 The endocrinology of late pregnancy. Estrogen production begins to fall just before parturition (arrow). Pregnanes slowly reduce over the last 4 weeks of pregnancy and decrease precipitously immediately at parturition. Progesterone gradually increases during the last month of gestation and then falls rapidly after foaling. Prostaglandin is significantly increased during stage 1 of labor with peak concentrations achieved during stage 2. Oxytocin peaks during stage 2 of labor immediately following the prostaglandin surge. Prolactin gradually increases over the last month of pregnancy, peaks at around 30–60 days after parturition and then tapers off over the next 2–4 months.

• There are high levels of fetally derived estrogen. The levels of estrogens produced by the fetoplacental unit and fetus remain relatively stable until parturition.

• Progesterone rises during the last few weeks of pregnancy with a peak about 5 days before foaling, whereas pregnane levels are decreasing (although they are still found at high levels).¹²

• Both progesterone and the 5α-pregnanes decrease precipitously following parturition and are at baseline levels within 24 hours of foaling.

• The ratio of the declining progesterone concentration to the stable estrogen provides the stimulus for the formation of oxytocin receptors and instigates changes in the muscular ability of the uterus to enable it to contract forcibly.

Relaxin is a hormone produced by the placenta ¹³ that stimulates relaxation of the pelvic ligaments in the birth canal and cervix. Concentrations of relaxin reach their peak during stage 2 of labor (fetal expulsion).

Prostaglandin F metabolites slowly begin to increase during the last third of pregnancy ¹⁴ and increase more significantly during the last 2 weeks. Prostaglandin, which is derived from the fetoplacental unit, reaches peak concentrations at the end of stage 1 of labor; this is partly responsible for cervical relaxation. Concentrations increase again at the onset of stage 2 of labor; this is partly responsible for the onset of coordinated uterine contractions.

Oxytocin is produced in a large surge once the fetus is positioned in the birth canal; this is probably the stimulus for the onset of stage 2 labor. Oxytocin production drops precipitously immediately after delivery. Thereafter it is produced in smaller surges during stage 3 to facilitate placental expulsion.

The ability of the uterine muscle to contract intensely is instigated by a combination of the drop in progesterone during the 24 hours prior to parturition followed by the repeated surges in prostaglandin and oxytocin production.

The onset of lactation is driven by the increasing production of prolactin from the anterior pituitary gland. Significant production is detectable during the last week of pregnancy.

PREPARATION FOR FOALING

Approximately 2 weeks before the anticipated due date, the mare should be moved to her foaling location. A daily and nightly routine should be established and followed without alteration so that she becomes accustomed to the routine and will not be disturbed by these normal occurrences when she is ready to foal.

The mare is remarkably capable of postponing active labor when she does not feel safe or secure. Most mares foal between 11 p.m. and 4 a.m., when there is the least amount of activity surrounding the mare.

• Disturbances in the mare’s environment may result in her delaying delivery until she feels comfortable with her surroundings.

• In this situation, the milk calcium level may be greater than 10 mmol/l and foaling may not occur within 48 hours as predicted by the kits (see p. 270).

The mare should be checked twice daily for signs of impending labor and possibly even more frequently as the signs of impending parturition develop. When the mare is very close to foaling she should be monitored frequently for signs of labor. Hourly or bi-hourly visits to the foaling stall/area should be made. For the most part interference should be minimized unless the mare is clearly in trouble. In order to minimize the extent of interference, supervision should be provided through some type of monitoring system (visual, video, alarm).

There are several different types of monitoring systems that can be utilized to minimize the number of physical visits and activity around the mare:

• Video monitoring systems can be set up in the mare stall, with remote TV monitors to evaluate foaling activity. These systems are unobtrusive, but are expensive and they require personnel to monitor the TV system.

• Belt/girth alarm systems, which are activated when abdominal contractions begin, can be strapped to the mare’s abdomen. They can be attached to a pager or phone system. False alarms may occur with these systems when the mare grunts or groans when in lateral recumbency.

• Transducers (or magnets) sutured to the vulvar lips are activated when the amniotic sac parts the vulvar lips. However, some mares are irritated by the transducers and will rub against a wall and tear the transducer out. These systems can be attached to a pager or phone system.

• A system utilizing a spirit level attached to the mare’s head collar which is activated when the mare lies in lateral recumbency is also available. This system relies upon the movement of the bubble to either end of the spirit level. There is plenty of scope for false alarms with this system when the mare simply lies down to rest.

In the late pregnant mare, some abdominal discomfort may be noted as a result of fetal position, fetal movement and pressure on other abdominal organs. Some term mares may show signs of abdominal discomfort during late gestation as a result of fetal positioning and increased fetal activity just prior to the onset of stage 1 labor.

• This discomfort may be recognized as colic by the owner. It is typically responsive to hand-walking, time, or administration of nonsteroidal anti-inflammatory agents.

As parturition approaches the mare will usually show a decrease in appetite.¹⁵ She may also try to separate herself from the rest of the herd if she is in a group.

Once the process of birth begins, the mare’s tail should be wrapped and the perineum washed with clean water, possibly with a mild soap, and then dried off.

If the mare has been previously subjected to a Caslick’s operation (Fig. 8.4) she should be examined in stocks, if possible twitched rather than tranquilized.

Fig. 8.4 Mares that have been subjected to a Caslick’s vulvoplasty operation should be restrained and the sutures removed if still present. A surgical episiotomy usually prevents uncontrolled and unpredictable perineal laceration and delays in delivery.

• After local anesthetic has been infiltrated along the previous incision line, the previously closed vulvar lips are sectioned to return the vulvar opening to its natural dimensions.

As far as possible foaling should not take place into a dirty, wet or muddy environment; stable facilities should be clean and free of any possible infectious agents, and foaling should occur apart from other foaling mares in particular. The management and hygiene involved in the preparation of the foaling box is described in Chapter 1.

• If the mare is to foal inside, a 6 × 6–7-meter loosebox is an adequate area. The mare should be carefully monitored to ensure that she does not foal into a corner.

• If the mare is to foal outside, there should be some shelter for the foal after birth. A clean, dry, grass pasture is ideal.

The flow chart presented in Fig. 8.5 provides a useful protocol for the decisions that need to be made at parturition.

Fig. 8.5 Decision flow chart for parturition.

STAGES AND SIGNS OF LABOR

Labor is classically divided into three stages (see Table 8.2).

Table 8.2

Signs and duration of the stages of labor

The umbilical cord will separate on its own either as the foal moves about in trying to rise or when the mare stands up. There is some difference of opinion about what to do with regard to the umbilical cord.¹⁶ Transfer of up to 1 liter of blood from the placenta back to the foal will occur until the cord breaks. This blood will pass both from the mare to the foal and from the foal to the mare. It is believed by some that this transfer of blood to the foal is important for its immediate well-being during the first few days of life. However, no significant differences in blood volume have been demonstrated between foals whose umbilical cords were allowed to remain patent for a quarter to half an hour after birth and those whose cords were severed and ligated immediately after delivery. Nevertheless, it is probably best if the umbilical cord is allowed to break naturally. If this occurs immediately after birth, it is not a reason for serious concern. If the mare and foal are both resting quietly, the cord should be allowed to remain intact until the actions of either the mare or the foal result in its spontaneous rupture.

After foaling, the mare should be allowed to rest. The process of labor is extremely strenuous for the mare, and she may lie quietly after foaling for up to 30–45 minutes in some cases. This time is often called the ‘period of tranquility’ and the mare should not be disturbed during this stage without good reason.

Stage 3

Stage 3 of labor is defined as the passage of the placental membranes and the onset of uterine involution (decrease in size and expulsion of fluid and debris). The duration of stage 3 is usually about 1–3 hours.

• Strong myometrial contractions may induce abdominal pain/discomfort (colic). The mare may lie down and get up repeatedly.

• If there is significant colic-like pain the mare may be hand-walked to prevent self-trauma. However, there are very important causes of colic in newly foaled mares (including cecal rupture, torsion of the large colon, and peritonitis) which should be eliminated before any decisions are made on treatment.

The placenta should be allowed to hang from the vulvar lips (see Fig. 8.11); however, if it is dragging on the ground or hitting the hocks it should be tied up, so the mare does not tear it off or kick at it and so injure the foal.

Fig. 8.11 Third-stage labor may be delayed. The placenta should not be forcibly removed, but it can be tied up to minimize the inconvenience. The placenta can be a source of serious infection for the foal.

Once the placenta has been delivered it should be placed in a bag and kept cool or refrigerated until it can be examined by a veterinarian (see p. 328). The veterinarian should examine every placenta in order to:

• Determine if it has been passed in its entirety.

• Evaluate it for abnormalities.

Uterine involution occurs quite rapidly in the mare.

• Within 12 hours after foaling, the uterus is usually only 1.5 times the size of its nonpregnant state.

• In the normal postpartum mare uterine involution is complete by day 15 (see p. 294).

PLACENTAL RETENTION

Failure to deliver the placenta within 2–4 hours after foaling is usually regarded as a ‘retained placenta’, but the specific circumstances may be more significant than the time.

In some cases the mare and the placenta will remain healthy for longer without problems, whereas in others there may be serious consequences from retention for less than 1 hour. For example, in Australia many mares that foal in paddocks rather than foaling boxes retain the placenta for 6–8 hours in the colder months without serious consequences.

• It is probably unwise, therefore, to dictate a specific time for placental delivery but rather to carefully assess the mare’s state of health and the character of the placental tissues.

In any case, a veterinarian should probably examine all mares that fail to expel the fetal membranes within 3 hours.

• In the event that a veterinarian cannot examine the mare immediately, advice on the possible use of 2-hourly intramuscular injections of 40 IU of oxytocin can be considered.

• There are important unwanted effects from this treatment so consultation with the veterinarian must be made before any treatment is given. In most countries the drug is only available on prescription.

When the mare is considered as being ‘not normal’, a veterinarian should be called to make further assessments regardless of the time period involved.

Attention should be paid to the health of the mare. In particular, she should be monitored for signs of anorexia (off-feed), depression, fever, or signs of laminitis (shifting weight, rocking horse stance, heat in the hooves and/or increased digital pulses). If any of these signs is present a veterinarian should attend immediately.

Management of retained placenta will be discussed later in this chapter.

POSTNATAL CARE

Care of the foal immediately after birth is critical for its well-being. The protocol for this is described on p. 365. A careful diary of events should be recorded.

The mare should be allowed some quiet time to bond with the foal immediately after delivery. During this time discreet observation can be maintained to ensure that no problems develop as a result of a nervous, apprehensive or aggressive mare, but the mare should not be disturbed until it is clear that there is a strong mutual bonding between the mare and foal. This is especially important for maiden mares or nervous mothers to ensure that abandonment will not occur. In the event that there are behavioral problems, the foal can be separated from the mare by a divider or the mare can be held by a familiar and experienced handler to allow the foal to approach and nurse. In some cases, the mare may need to be sedated before it will allow nursing until the pressure on the full udder decreases and the mare becomes accustomed to the feel of the nursing foal. On rare occasions, rejection of the foal may occur. A nurse mare may be required or the foal may need to be to raised as an orphan. The latter is to be avoided at almost any cost.

After bonding has taken place the mare should be examined for injuries such as perineal lacerations, perforations of the vaginal/digestive tract, or hemorrhage. The mare should be monitored closely for the first 12 hours after foaling, because some internal injuries may not be apparent for several hours. Where necessary vulvoplasty (Caslick’s suture) should be carried out as soon as possible.

The mare’s udder and perineum (including the thigh regions) should be cleaned and an assessment of both colostral quality and quantity should be made before the foal nurses.

INDUCTION OF PARTURITION

There is no question that the best place for a fetus to be is in the mare’s uterus. It has been said that the fetus picks the day it will be born and the mare picks the hour it will be born. Induction of a delivery where the fetus is not yet mature may result in a weak, premature foal that requires intensive care or the birth of a stillborn or a dead fetus.

• Failure to ensure that the foal is indeed ready for birth and that the mare is in a suitable physiological and psychological state can result in considerable problems.

An induced delivery (and particularly where this is premature) can result in critical problems for both the mare and the foal including:

• Premature placental separation; this is common with induced parturition.

• The presence of a red, velvety structure at the vulvar lips (see p. 298) or palpation of the chorioallantois lying in the uterine lumen are both indications for emergency manual rupture of the placenta and IMMEDIATE delivery of the fetus.

• Failure of passive transfer (p. 375).

• Dystocia (p. 379).

• Fetal or maternal compromise and death.

• Retained placenta (p. 313).

• Neonatal maladjustment syndrome. This is quite common in foals born by induction even when at full term, and immediate (and often long-term) care is often required for their survival.

There are considerable advantages in an induced parturition but also significant dangers. The procedure should not be undertaken lightly.9–11 Induction performed by the attending veterinarian provides continuous professional care and monitoring for the mare and the foal. Problems such as premature placental separation and fetal malposition/dystocia can quickly be recognized and dealt with without delay. Furthermore, careful and full preparations can be made in advance and the tendency for mares to foal in the early hours of the morning can be negated.

There are basic rules that can be used to ensure that an induced delivery has the best chance of producing a normal foal with minimal complications to both foal and mare. Careful and thorough assessment of the mare and the fetus are essential if problems are to be avoided rather than created. Mistakes can easily result in the delivery of a nonviable foal.

Note:

• It is extremely important that the owner is made aware of the potential dangers, particularly to the possibility that foal pathology may already be present in utero and may result in a nonviable foal which may be incorrectly attributed to the induction process.

It is therefore important to select the cases for induced delivery very carefully based on the whole circumstances and to ensure that the procedure is followed through completely. The decision to induce should be based on specific needs rather than purely on convenience.

Emergency medications and oxygen should be made available in the event that there is a problem. The owner and veterinarian should be fully prepared for the delivery prior to the onset of induction. All the necessary intensive care, resuscitation equipment and obstetric facilities should always be directly available.

Indications for induced delivery

The indications for induced delivery include:

• A classification as a high-risk foaling as a result of mare or foal classification.

• History of difficult deliveries or previous abnormal or problematical foals; thus foaling or situations in which the life of the mare or foal is likely to be in jeopardy can be supervised.

• Habitual or anticipated premature placental separation (‘red bag’ delivery).

• History of primary uterine inertia resulting in delayed delivery.

• Inability to strain effectively (abdominal muscle, diaphragmatic or tracheal defects).

• Physical deformity of the maternal pelvis, or with a history of previous pelvic injury including old fractures, such that attended labor is critical, impending rupture of the prepubic tendon, or uncontrolled and recurring prefoaling colic. The decision must include the possibility of cesarean section rather than induction of parturition.

• Detectable fetal distress or other life-threatening injury or disease in the foal or the mare.

Note:

• Mares may also be induced for use as nurse mares.

• Inductions should not be performed solely for the convenience of owner or veterinarian.

Criteria for decision

There are five ‘rules of thumb’ to be followed to provide the best chance for a successful induction:

• There should be colostrum in the udder.

• Gestational age must be between 330 and 350 days. Although the actual gestational age is important, the previous history of the individual mare might suggest that a delay or an earlier induction might be indicated. Thus it is probably not wise to assume normality in a primiparous mare and an assumption based upon the ‘normal’ gestational duration of the horse is most unwise. Optimal survival is achieved by induction within 10 days of normal delivery date for that pregnancy.

• Pelvic relaxation should be present. The cervix should be relaxing/dilating (as detected by manual examination or vaginoscopy).

• The fetus should be in a normal presentation, position, and posture.

• Mammary secretion analysis is consistent with readiness for birth. The milk calcium level should be >10 mmol/l (>200 ppm).^5,²¹ A system that provides a score derived from calcium, sodium, and potassium can be used if each of these can be measured accurately (see Table 8.3).²² This usually requires a laboratory that is equipped to perform specialized testing procedures. It is often worth measuring the IgG concentration if the milk calcium has risen sufficiently to permit induction to determine if passive transfer will be normal following delivery. This measurement should be performed again after the foal has been delivered. Assessment of fetal maturity by evaluating mammary secretion concentrations of calcium, sodium, and potassium is described on p. 270.