Chapter 22 PARTURITION AND EVALUATION OF THE PLACENTA
Various aspects of late gestation in the mare make horses unique among the domestic species. Gestation length is extremely variable, with the range for normal gestation length in horses encompassing a 6-week period and a reported range of delivery of normal healthy foals of 305–405 days. In no other species is the concept of “readiness for birth,” as suggested by Rossdale and colleagues,1,2 so apparent. Fetal maturation is important in timing the onset of parturition in most species, but variability is expressed in days, not weeks.
HORMONAL CHANGES
Increasing estrogens and declining progestogens at the end of gestation are characteristic of many domestic species. In horses, however, estrogens decline and progestogens rise in the last weeks of gestation, followed by a rapid decline in progestogens in the last days before delivery. Abnormal progestogen concentrations may signal placental disease (see Chapters 20 and 21). A precocious rise in progestogens is often observed with placentitis. Ousey et al.3 reported that increased progestogens were found in mares with placentitis compared with mares with normal pregnancies, indicating increased fetal production or increased utero-placental metabolism in response to the chronic stress. Leblanc et al.4 and Stawicki et al.5 reported similar findings in a model of experimental placentitis. Conversely, an early decline in progestogens is usually associated with acute stress and abortion.4
MILK ELECTROLYTES
It is clear that a metric for fetal maturation is needed to be able to predict when parturition will occur or when intervention can be done safely. Peaker et al.6 measured components of mammary secretions pre-partum and found that calcium rose to >10 mmol/L 1–6 days before foaling. This was followed by reports supporting the finding of a rapid pre-partum rise in calcium in mammary secretions.7,8 Interestingly, a similar abrupt rise in calcium occurs in allantoic fluid shortly before parturition.9 Not only does milk calcium rise rapidly, but the relative concentrations of sodium and potassium invert, with potassium concentrations becoming greater than sodium in the last few days of gestation.
Based on these characteristic changes, mammary secretions were deemed to be a very good way to assess fetal maturity or “readiness for birth.”7,8 Foals born from mares induced to foal when calcium was <10 mmol/L had a poorer chance for survival compared with foals induced when calcium was >10 mmol/L.7,8
Water hardness kits are useful for determining the concentration of calcium in mammary secretions.10–12 However, the use of water hardness test kits is not without potential problems. Many kits test for divalent cations, which includes magnesium as well as calcium. Magnesium begins to increase earlier than calcium, and the rise is more gradual. Because of this slower, earlier rise in magnesium, water hardness tests that do not differentiate between magnesium and calcium complicate interpretation of results. Furthermore, the peak concentration of magnesium is reached earlier than is calcium, and magnesium often declines at parturition.12,13 For these reasons, if a test is used to decide when to induce parturition, it is critical that the test measure only calcium. If the intent is merely to determine the likelihood of the mare foaling on a particular night, the type of test is not as important; however, those that test for only calcium will have a better predictive value.
Another potential confounding factor in the interpretation of milk calcium test results is that in the presence of placentitis or with twin pregnancy, there is often a premature rise in calcium. Elevated milk calcium before 310 days has been suggested to be an indication of an abnormal pregnancy.14 Obtaining additional information by determining the concentration of sodium and potassium, in addition to calcium, is recommended if possible. If calcium is greater than 400 ppm, yet sodium remains greater than potassium, parturition should not be induced because a high probability of placentitis or undiagnosed twins exists.
Given that 10 mmol/L (400 ppm) calcium in mammary secretions has been well established as being a good indicator of fetal readiness for birth,6–813 test results should be interpreted while keeping this standard in mind. Some manuscripts have referred to 200- or 250-ppm calcium carbonate as benchmarks for readiness for birth.15,16 However, these values carry potential danger if used for induction of parturition. Calcium in milk is not in the form of calcium carbonate. Results from tests that measure calcium carbonate in a solution should be converted to calcium for correct interpretation. To convert ppm calcium carbonate to ppm calcium, results should be divided by 2.5 because the molecular weight of calcium carbonate is 100 and the molecular weight of calcium is 40 (100/2.5 = 40); ppm is derived by multiplying mmol × molecular weight). Therefore, 200- and 250-ppm calcium carbonate are equivalent to only 80- and 100-ppm calcium, respectively, or 2- and 2.5-mmol calcium carbonate or calcium (mmol are not dependent on molecular weight).
Here is an example calculation:
In the manuscript using 200 ppm calcium carbonate (uncorrected results), as a predictor of foaling,16 if the value is corrected for dilution and converted to calcium, the benchmark used is actually 560 ppm calcium, equivalent to 14 mmol/L; this is in agreement with the earlier reports.7,8,12
When used for estimating the likelihood of a mare foaling on a given night, milk calcium tests are more reliable for judging when a mare is not likely to foal than for predicting when she is likely to foal.12 Of course, one reason for this is a mare’s apparent ability to control when she goes into labor. It is a common observation that mares can seemingly withhold the onset of labor until surrounding conditions are suitable.17 It is not unheard of for a mare with a milk calcium >10 mmol/L that is moved to a new environment where there is a lot of activity to not foal for days.
