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Reason for performing study: An understanding of the normal events of foaling, causes of dystocia and clinical outcomes is important for equine practitioners.
Objectives: The goals of the present study were to: 1) evaluate factors that influence gestation length; 2) report duration of Stage II labour; 3) determine the frequency of dystocia and premature placental separation; and 4) determine the relationship between problems at foaling and foal survival.
Materials and methods: Foaling records of 1047 mare births were evaluated.
Results: The average gestation length was 341 ± 0.3 days, with no effect of mare age or breed observed. Mares carrying male fetuses had a longer gestation (P≤0.001) than mares carrying female fetuses. A majority (52.8%) of mares foaled at night between 2000 h and 0200 h when the facility was quiet. Dystocia occurred in 10.1% of all births and the incidence rate was higher in Thoroughbred mares than in Quarter Horse mares. The most common cause of dystocia was abnormalities of fetal posture. A delay in foal delivery beyond 40 min of Stage II of labour was associated with a significant increase in foal mortality. In addition, an increase in foal morbidity and mortality was noted when the interval from birth to standing or birth to nursing was prolonged.
Conclusion: Early detection and rapid appropriate intervention are critical to foal survival in an equine dystocia.
Potential relevance: Equine veterinarians should counsel horse owners that early recognition of a foaling problem and rapid, appropriate intervention are critical to the survival of a foal.
The perinatal period is a critical stage in the life of a foal. Decisions made at the time of foaling may dictate the future health and survival of the foal. Consequently, an understanding of the normal time course and progression of foaling and problems that occur during the foaling process are of paramount importance to veterinarians, horse owners and foaling attendants.
Dystocia or difficult birth occurs in approximately 4% of Thoroughbred foalings (Rossdale and Ricketts 1974). Abnormal orientation of the fetus in the birth canal (i.e. postural abnormalities) is the most common cause of dystocia in the horse (Rossdale and Ricketts 1974; Roberts 1986). Early recognition and intervention during a dystocia may be required to save the life of the foal, save the life of the mare and preserve future fertility of the mare. A recent retrospective study of dystocias at a referral hospital indicated that each 10 min increase in the duration of Stage II labour beyond 30 min was associated with a 10% increase in the existing risk of a foal being born dead and a 16% increase in risk of the foal not surviving to discharge from the hospital (Norton et al. 2007).
The objectives of this retrospective study were to: 1) evaluate factors influencing gestation length; 2) report duration of Stage II labour, as well as intervals from birth to when the foal first stood, nursed and passed meconium; 3) determine the frequency of dystocia and premature placental separation; and 4) determine the relationship between problems at foaling and foal survival.
Materials and methods
Medical records from 2002 to 2010 were obtained from an equine facility that specialises in attending foaling of Thoroughbred and American Quarter Horse mares (Hunter Creek Stables, Roswell, New Mexico, USA). Data were collected prospectively and recorded for mare age, breed, gestation length, sex of foal, time of foaling, interval from rupture of the chorioallantoic membrane to delivery, interval from birth to when the foal first stood, nursed and passed meconium, occurrence and type of dystocia, premature separation of the placenta, and health of the foal (presence or absence of lethargy, fever, anorexia, diarrhoea etc. as assessed by the on-farm foaling attendant). Although all data lines were filled out not all data were deemed suitable for analysis, hence the sample size changed for each category. For example, if rupture of the chorioallantoic membrane was unobserved, the foaling attendants listed the event as occurring ‘by ••.•• time’, this was deemed to be an estimate and the data were excluded from analysis. Additionally, only mares with a normal gestation and pregnancy were included in the analysis, whereas 6 mares with placentitis, one with potential hydrops allantois and one with severe laminitis associated with a premature delivery were excluded from analysis.
In general, mares were housed outdoors in large paddocks and exposed to an ambient photoperiod until they approached their due date. Mares were then brought inside a barn and housed in large foaling stalls with constant light exposure to allow for observations by foaling attendants via a closed-circuit camera system.
The foaling attendant provided assistance during dystocia. Dystocia was defined as prolonged Stage II of labour (>20 min) or a cessation of progress of the foal through the birth canal. A veterinarian was called if the foaling attendant could not determine the cause of the dystocia, could not bring the fetus into normal presentation, position or posture, or could not deliver the fetus through the birth canal. The interval from initial contact to arrival of the veterinarian was approximately 10–15 min. Oxygen was administered via an intranasal tube at a rate of 5 l/min to foals that were born after a prolonged Stage II of labour (>30 min) or showed clinical signs suggestive of hypoxia (unconscious, nonresponsive or evidence of cyanosis in the immediate neonatal period).
Stillbirth was defined as a foal born dead that had never taken a breath and never had a detectable heart beat. Morbidity was defined as disease or medical condition affecting a foal within the first 5 days of life. Mortality was defined as death within the first 5 days following delivery of a foal that had been born alive. Survival was defined as the foal being alive and healthy 5 days after birth, with no overt sickness (i.e. diarrhoea, depression, lethargy etc.). Stage II of labour was defined as the time from rupture of the chorioallantoic membrane to passage of the fetus. Premature placental separation was defined as passage of the fetus within the chorioallantoic membrane without rupture of the chorioallantoic membrane.
Statistical analysis was performed using a commercial software package (SAS)a. The numbers of mares foaling in different 4 h time intervals (1000–1400 etc.) were compared using one-way analysis of variance. The rate of still births, morbidity and mortality for mare age, breed, sex of fetus, duration of Stage II of labour, and interval from birth to standing and nursing were compared using Chi-squared analysis. Gestation length and the interval for Stage II of labour, standing, nursing and passing meconium were log transformed and compared by least squared means: data presented were back transformed for representation. Data are presented as mean, upper and lower 95% confidence intervals, and range. Significance was set at P<0.05.
A total of 1047 foalings from 652 individual mares (35% Thoroughbred, 41% American Quarter Horse, and 24% other/unknown) with an average age of 10.3 years (range 3–29 years) were recorded and evaluated. The average gestation length was 342.7 days (95% CI 338.5–346.8; range 307–381). Mares foaling in January had a significantly shorter (P<0.05) gestation length (n = 133; 338.1 days; 95% CI 335.3–340.9; range 306–358) than mares foaling in February (n = 212; 341.9 days; 95% CI 339.4–344.4; range 316–372), March (n = 313; 342.0 days; 95% CI 339.6–344.4; range 315–375), April (n = 289; 341.7 days; 95% CI 339.1–344.1; range 307–381) and May (n = 93; 342.3 days; 95% CI 339.2–335.4; range 322–367). Mares that gave birth to a colt had a significantly longer gestation (n = 541; 344.0 days; 95% CI 340.9–347.1; range 320–360) than mares that gave birth to a filly (n = 500; 341.3 days; 95% CI 338.2–344.4; range 306–381). A significant difference in gestation length for breed and mare age was not observed.
A live foal was produced in 1030 of the pregnancies ranging in duration from 311–381 days. Approximately 94.4% (988 of 1047) of mares foaled between 320 and 360 days of gestation, whereas 1.1% (12 of 1047) foaled prior to Day 320 and 3.9% (41 of 1047) had a gestation length of >360 days. The morbidity and mortality of foals born <320 days, 320–360 days and >360 days of gestation were not significantly different (Table 1).
Table 1. Stillbirth, morbidity and mortality of foals with known gestation length
No significant differences were noted within a column.
A significantly larger percentage (P<0.05) of mares foaled from 20.00–06.00 h (72.7%; 729 of 1002) than from 06.00–20.00 h (27.3%; 273 of 1002) (Fig 1). The majority of mares (52.8%; 529 of 1002) foaled within a 6 hour period from 20.00–02.00 h. Relatively few foals (13.4%; 134 of 1002) were born during daylight hours (08.00–17.00 h).
The average length of Stage II of labour was 16.7 min (95% CI 14.5–19.0; range 2–152 min) with 71.7% (721 of 1005) being born in <20 min. The mean duration of Stage II of labour was not affected (P>0.05) by mare age, sex of the foal or breed. A significant increase in still birth, foal morbidity and foal mortality occurred when the duration of Stage II was >40 min (Table 2). In a vast majority of mares (n = 975) Stage II of labour was ≤40 min. The percentages of stillbirth, foal mortality and foal morbidity for Stage II of labour of ≤40 min duration were 1.2% (n = 12), 3.1% (n = 30) and 4.2% (n = 41), respectively. Only 30 mares had a duration of Stage II labour >40 min. However, this was associated with a significant increase in still births (17%; n = 5), foal mortality (20%; n = 6) and foal morbidity (10%; n = 3). The odds ratios (OR) for stillbirths, mortality and morbidity in foals with Stage II of labour >40 min was 16.1 (5.3–49.0), 7.9 (3.0–20.7), and 2.5 (0.7–8.7), respectively.
Table 2. Stillbirth, foal morbidity and foal mortality associated with duration of Stage II of labour
Duration of Stage II
A difference in superscripts within a column indicates a significant difference (P<0.05).
An unassisted delivery occurred in 87.3% (914 of 1047) of foalings. Dystocia occurred in 10.1% (106 of 1047). The majority of dystocias were associated with a foal that was in cranial presentation (96.2%; 102 of 106) and dorsosacral position (78.3%; 83 of 106); foal survival associated with these orientations was 94 of 102 and 75 of 83, respectively. A dorsopubic position was associated with 16.9% (18 of 106) of all cases of dystocias; foal survival was 18 of 18. Abnormalities of fetal posture were associated with 37.7% (40 of 106) of dystocias, including one or both front limbs retained (30.1%; 32 of 106), retained fetal head (3.8%; 4 of 106) or both limb and head retained (3.8%; 4 of 106). Foal survival for these posture abnormalities was 30 of 32, 1 of 4 and 2 of 4, respectively. A total of 8 cases (0.8% incidence rate) of ‘hip-lock’ (fetopelvic disproportion) were recorded. Five of the 8 foals survived. A significantly higher (P = 0.015) number of dystocias were observed in Thoroughbred mares (13.7%; 54 of 395) than in Quarter Horse mares (7.9%; 36 of 456).
Premature placental separation was recorded in 1.6% (17 of 1047) of foalings. All of the foals were born alive and had a mortality rate of 17.6% (3 of 17) and morbidity rate of 0%.
Eight mares (0.8%; 8 of 1047) had a severe haemorrhagic episode in the periparturient period; 3 of these mares did not survive. The 8 mares with a haemorrhagic event had apparent normal foaling events with a Stage II labour <20 min in duration. Three mares (0.3%; 3 of 1047) sustained a uterine tear; one of these was associated with a severe dystocia (cranial, dorsalsacral presentation, with a leg flexed at the carpus and the head turned). An additional 4 mares (0.4%; 4 of 1047) sustained a severe perineal laceration during foaling.
Post partum period
The average interval from delivery of the fetus to when the foal stood unassisted was 49.2 min (95% CI 39.7–56.4 min; range 6–1019 min). Colts took significantly (P<0.05) longer to stand (54.1 min, 95% CI 46.5–62.9 min) than fillies (44.2 min, 95% CI 37.9–51.5 min). Quarter Horse foals stood significantly (P<0.05) sooner (43.1 min, 95% CI 37.1–50.0 min) than Thoroughbred foals (55.4 min, 95% CI 39.7–63.7 min). A significant difference in the interval from birth to standing was not observed for mare age. Foals that required more than 60 min to stand had a significant increase (P<0.05) in morbidity and mortality (8.4%, 23 of 273; OR 3.8 [1.4–10.0] and 3.7%, 10 of 273; OR 3.7 [1.9–7.1], respectively) compared to foals that stood unassisted within 60 min (2.4%, 17 of 704 and 0.9%, 7 of 704), respectively).
The average interval from birth to nursing was 111.7 min (95% CI 92.3–125.3 min; range 15–1200 min). Compared to fillies (103.2 min, 95% CI 89.6–118.7 min) colts took significantly (P = 0.0008) longer to nurse (119.7 minutes, 95% CI 104.2–137.5 min). Quarter Horse foals nursed in a shorter time (101.0 min, 95% CI 88.9–116.8 min) (P = 0.0002) than Thoroughbred foals (121.2 min, 95% CI 104.9–139.8 min). No significant (P>0.05) affect of mare was observed on interval to nursing. A significant increase (P<0.05) in morbidity and mortality was observed in foals that took >120 min from birth to nursing (7.7%, 26 of 338; OR 3.5 [1.0–11.7] and 2.4%, 8 of 338; OR 3.9 [1.9–7.9], respectively) compared to foals that nursed within 120 min (2.1%, 12 of 579 and 0.7%, 4 of 579, respectively).
The average interval from birth to passage of meconium was 30.8 min (95% CI 23.5–42.7 min; range 3–1433 min). There was no statistical difference in the interval for passage of meconium by sex of the foal, mare age or breed.
The majority of reports have indicated that mares bred to foal during winter (short duration of daylight) have a longer gestation period than mares bred to foal during summer (longer duration of daylight) (Howell and Rollins 1951; Hodge et al. 1982; Marteniuk et al. 1998). Results from this study showed a significant decrease in duration of gestation for foals born in January. Mares were brought into the foaling barn from outdoor paddocks as they approached their foaling date; mares foaling in January would have been influenced for a substantially longer period by an artificial photoperiod during late gestation than those foaling in any other months. Hodge et al. (1982) demonstrated that maintenance of Quarter Horse mares under an extended photoperiod regimen (16 h light:8 h darkness) significantly shortened gestation length compared to control mares maintained under an ambient photoperiod.
In this study the average interval for Stage II labour was 16.7 min. A prolongation of active Stage II labour beyond 40 min was associated with significantly fewer foals born alive or surviving to age 5 days in the present study. Norton et al. (2007) recently reported that every 10 min increase in the duration of Stage II labour beyond 30 min was associated with a 10% increase in the existing risk of a foal being born dead, and a 16% increased risk of the foal not surviving to discharge. Byron et al. (2002) further noted that the interval from rupture of the chorioallantoic membrane to delivery of the fetus was 13.6 min shorter for foals that were born alive and survived to discharge than for foals born dead or that did not survive to discharge. Together, these studies reinforce the concept that early accurate detection of a foaling problem and early effective intervention is critical to foal health and survival.
In the present study, a dystocia rate of 10.1% was noted for the 1047 births. Ginther and Williams (1996) reported an overall dystocia rate of 11.2% for 517 spontaneous parturitions on a group of 8 equine breeding farms. Malposture of fetal extremities (41.4%) and malposition of the body (22.6%) were the most common abnormalities of fetal orientation reported in that study. A normal fetal position and posture was reported in 31.0% of dystocias, and in those cases the dystocia was commonly associated with a large fetus or weak maternal contractions. In the present study 36% (38 of 106) of dystocias were associated with a normal fetal position and posture, similar to the results of the study by Ginther and Williams (1996). In the present study, abnormality of fetal posture, especially retention of the front legs and/or head, was also the most common cause of dystocia. Retrospective studies of equine dystocia cases presented to referral hospitals also indicate postural abnormalities of the head and limb as the most common causes of foaling problems (Frazer et al. 1997; Byron et al. 2002).
We did not anticipate the significant difference in the number of dystocias between Thoroughbred mares (13.7%) and Quarter Horse mares (7.9%) and do not have an explanation for this observation. Others have reported breed differences in dystocia, but related primarily to differences in dystocia rates between Thoroughbred and draught horse breeds (Frazer et al. 1997). The dystocia rate of Thoroughbred mares in the present study was higher than the 4% reported by Rossdale and Ricketts (1974).
Premature separation of the placenta, also known as ‘red-bag’, was observed in 1.6% of foalings in the present study and 2.0% of foalings reported by Ginther and Williams (1996). Failure of the chorioallantoic membrane to rupture during labour and subsequent separation of the microcotyledonary attachments between the uterus and fetal membranes lead to a rapid decrease in oxygen transport to the fetus. As a consequence, the fetus may suffer from hypoxia or may die of asphyxiation if the condition is prolonged or progresses. Early recognition of premature placental separation and immediate intervention by rupture of the chorioallantoic membrane and delivery of the foal are key factors to survival of the foal with this emergency condition.
The intervals from birth to when the foal initially stood unassisted (49.2 min; range 6–1019), nursed (117.7 min; range 15–1200), and passed meconium (30.8 min; range 3–1433) observed in the present study are close to the time-honoured ‘1-2-3 Guidelines’, which suggest that foals should stand by 1 h, nurse by 2 h, and pass meconium by 3 h. Although time to standing and nursing follows the suggested guidelines, average time to passing meconium does not typically take 3 h. There was a significant increase in foal morbidity or mortality if the interval from birth to standing and birth to nursing was >60 or 120 min, respectively.
In conclusion, early detection of an equine dystocia and rapid appropriate intervention in problem cases are critical for foal survival. A delay in delivery after rupture of the chorioallantoic membrane beyond 40 min was associated with a decrease in foal survival. In addition, failure of a foal to stand within 60 min and nurse within 120 min was also associated with an increase in foal morbidity and mortality.
Conflicts of interests
No conflicts of interest have been declared.
Sources of funding
The authors would like to thank Susan Hunter and the staff at Hunter Creek Stables for diligently recording foaling data and allowing the data to be published.