CHAPTER 62Normal Prefoaling Mammary Secretions

Normal gestational length can vary widely in the mare. The majority of mares foal during “nonbusiness” hours for most horse owners, as well as veterinarians. Thus many sleepless nights can be spent waiting up for the mare to foal. A 24-hour distribution of spontaneous foaling times for over 1,100 mares is depicted in Figure 62-1.¹ Physical signs of the mare’s approaching readiness for foaling include a gestation length of greater than 320 days, udder enlargement with the presence of colostrum or milk in the teats, waxing on teat ends, and relaxation around the tail head, buttocks, and lips of the vulva. Although helpful, none of these signs is extremely accurate as a means of predicting when the mare will actually foal.

Figure 62-1 The distribution in a 24-hour period of spontaneous foaling times for over 1,100 mares.

(Modified from Bain AM, Howey WP: Observations on the time of foaling in Thoroughbred mares in Australia. J Reprod Fertil Suppl 1975; 23:545)

Studies of prefoaling milk (mammary secretion) electrolyte changes have demonstrated an association with the mare’s approaching readiness to foal.²^–¹¹ These electrolyte changes, especially with regard to calcium, have also been related to the development of maturity of the foal in the uterus and its subsequent survivability (i.e., viability) following a normal delivery.^4,^8,^10,^12,¹³

Prefoaling mammary secretion electrolytes are related to fetal readiness for birth.⁴ Calcium carbonate (CaCO₃) content of such samples has been shown to be both sensitive and specific.¹⁰ The predictive value of a positive test (PVPT) (defined as the first occurrence when CaCO₃≥ 200 ppm) was determined to be 97.2%, for normal pregnant mares foaling spontaneously within 72 hours. Mares with placentitis and other causes of precocious lactation, however, have early elevation of mammary secretion calcium.¹⁴ Before 310 days’ gestation, elevated milk calcium levels indicate placental abnormality and not fetal maturity.

It has been suggested that the inversion of sodium and potassium concentrations in prefoaling mammary secretion does not occur in mares experiencing placentitis and may be a better indicator of in utero fetal maturation.¹⁵ However, no published reports could be found to support this hypothesis other than the very early work of Peaker et al and Ousey et al.^2,⁴ These both used normal, spontaneously foaling mares for their investigations. We repeated this work (a) to evaluate prefoaling mammary secretion sodium and potassium concentrations and (b) to document the time of their relative inversion before foaling in normal mares. A consistent pattern could not be demonstrated¹⁶; 5 of 14 mares foaled normally, delivering live, healthy foals, and none demonstrated a sodium to potassium electrolyte inversion on any sample collected daily or twice daily from 7 to 10 days before spontaneous parturition. In a more recent work, it was reported that concentrations of potassium, calcium, citrate, and lactose increased and concentration of sodium decreased as foaling approached but that variation between mares was large.¹¹ These authors concluded that the use of prepartum equine mammary secretion electrolyte concentrations for prediction of time of foaling is unreliable due to the large variation in both absolute and change in electrolyte concentrations between mares. This is absolutely correct. None of these methods has yet been shown to predict time of foaling. Ley et al¹⁰ stated that prefoaling mammary secretion CaCO₃testing is a helpful prognostic tool to indicate the mare’s approaching readiness for birth and a method to predict when the mare is not likely to foal within 24 hours when the CaCO₃is less than 200 ppm. Ousey et al⁴ suggested by their results that fetal maturity may be related to electrolyte concentration in mammary secretion and that an ionic score greater than or equal to 35 may indicate that induction would be successful in terms of maturity of the newborn foal. Leadon et al³ stated that calcium concentrations of the mammary secretions proved useful in predicting full term and also in assessment of the chances of foal survival in prematurely induced parturition. Ousey et al⁹ in a later report stated that mammary secretion electrolyte testing before foaling was not particularly accurate in predicting time of parturition, although it was a reliable means of indicating when it was not necessary to attend prepartum mares at night. It is reasonable to agree with Douglas et al¹¹ that these tests do not predict foaling time. Very few investigations have actually claimed that they do. Prefoaling mammary secretion electrolyte testing methods are simply a means to evaluate in utero fetal maturation and the approaching readiness for birth.