The mare should be taken to the foaling location 3–4 wk before the expected foaling date to enhance immunity against pathogens present in the environment. Antibodies will be sequestered in colostrum for passive immunity in the newborn.
Foaling box stalls should be large (at least 3.5 × 3 × 3.5 m). The foaling area should have good ventilation and be well bedded with clean, dry straw. The walls should be solidly constructed and free of sharp edges. Mares may benefit from foaling outdoors on turf, weather permitting. Regardless of location, the mare should be observed without disturbance.
Evaluation of the premonitory signs of parturition is useful but does not permit precise prediction of the time of delivery. The mammary gland starts developing 3–6 wk before foaling and distends with colostrum in most mares 2–3 days before parturition. Colostrum may drip from the teats and dry to form a waxy material at each teat orifice. This “waxing” develops in ~95% of mares 6–48 hr before foaling, but in some cases, it is not seen at all or it may precede parturition by many days. Before the mare foals, the calcium and potassium content of udder secretions increase, and the sodium content decreases. Water hardness chemical tests have been used on mammary gland secretions to predict imminent parturition.
Stages of Parturition
It is critical to understand the normal progression of events during parturition. This permits identification of abnormal events and will help determine when intervention is needed. Parturition is divided into 3 stages.
Stage I is characterized by signs of abdominal pain and restlessness due to uterine contractions. Patches of sweat on the flank and behind the elbows usually appear a few hours before foaling. The uterine contractions increase in frequency and intensity, causing the fetus to engage into the cervix and pelvic canal. The fetus rotates from a dorsopubic to dorsosacral position before expulsion. Mares may roll during the first stage, which is thought to facilitate rotation of the fetus.
Increasing pressure in the uterus causes the allantochorion to protrude through the internal os of the cervix. The allantochorion over the cervix does not have microvilli and is referred to as the cervical star. The allantochorion usually ruptures at the cervical star, and the release of the tea-colored allantoic fluid (“water breaks”) marks the end of the first stage of parturition.
Stage II starts with the rupture of the allantochorion and ends when the fetus is expelled. Second stage labor usually lasts 10–30 min. Stimulation of the cervix by the fetus (Ferguson reflex) causes the mare to have abdominal contractions. The allantoic fluid lubricates the canal, facilitating expulsion of the amnion and fetus. Vaginal distention causes release of oxytocin and further myometrial and abdominal contractions. The amnion appears at the vulvar lips as a whitish, fluid-filled structure. The straining efforts of the mare consist of 3–4 strong contractions, followed by a short period of rest. The mare usually assumes lateral recumbency during delivery with limbs extended.
The foal is normally delivered in an anterior, longitudinal presentation and dorsosacral position with the head, neck, and forelimbs extended. One front hoof of the foal usually precedes the other hoof by ~15 cm, facilitating passage of the elbows and shoulders through the pelvic canal. The foal is usually born with the umbilical cord intact and covered by the amnion, which is ruptured by movements of the mare or the foal. If amnion remains over the foal's nose, an attendant should remove it to prevent suffocation. If left undisturbed, the mare may lie for some time with the foal's hindlimbs in the vagina. If the foal has not been delivered within 30 min of the rupture of the chorioallantois and release of the allantoic fluid, obstetric intervention is warranted.
Stage III involves expulsion of the fetal membranes. Normally, fetal membrane passage occurs rapidly (within 3 hr) after delivery of the foal. The weight of the amnion and cord may help the allantochorion separate from the endometrium. Progressive traction by the amnion and moderate uterine contractions originating at the tip of the horn cause complete separation of the allantochorion, which may become inverted during the process. The mare may stand with the amnion hanging from the vulva at the level of the hocks or below. If the mare kicks at the hanging membranes, which endangers the foal, the exposed membranes should be tied in knots to shorten the length above the hocks.
If the fetal membranes have not been passed by 3 hr after parturition, oxytocin (20 IU, IV or IM) should be administered at 15- to 30-min intervals until membranes pass. If fetal membranes are still retained at 8 hr postpartum, therapy for retained placenta should be instituted.
Premature Separation of the Placenta
Normally, the translucent white amnion appears first at the vulvar lips during the expulsion stage of labor. Premature separation of the placenta is characterized by the appearance of the bright red, velvety, intact chorioallantois between the vulvar lips before the foal is delivered. The presence of the chorion at the vulvar lips indicates that it has separated from the endometrium before the foal is able to breathe spontaneously. The chorioallantois must immediately be ruptured and the foal manually delivered, or the foal will asphyxiate. The severity and duration of hypoxia that results determines the severity of neurologic abnormalities displayed by the foal (peripartum asphyxia or hypoxic ischemic encephalopathy [see Hypoxic Ischemic Encephalopathy]).
Last full review/revision July 2011 by Patricia L. Sertich, MS, VMD, DACT