How to Maintain Pregnancy

1. Introduction

On some days, “how to maintain pregnancy” seems an elusive goal. In some cases, by the time practitioners recognize that a mare is demonstrating signs of preterm birth, any assistance we can provide is insufficient to allow delivery of a vigorous and healthy foal. Thus, a large component of how to maintain pregnancy is determining when to intervene.

Bosh et al (2009) described reproductive performance measures among 1011 Thoroughbred mares in central Kentucky during the 2004 mating and 2005 foaling seasons (Table 1). This study demonstrated a 12.9% pregnancy loss between day 40 of gestation and foaling.¹
 

This is similar to fetal loss rates between 3 and 10 months gestation found in other studies.²  In comparison, the human rate of preterm births in the United States in 12.5%.³

In a review of 3527 aborted fetuses and placentas, stillborn foals or foals that died <24 hours after birth, 34% were due to feto-placental infection or placentitis of unidentified etiology. Bacterial infection caused 628 (17.8%) of these losses. Complications of birth led to the demise of 679 (19%), congenital anomalies including contracted foals counted for 348 (10%), and no diagnosis was identified in 585 (17%) of these cases. Other causes included placental edema, premature placental separation, twins, umbilical cord anomalies, placental villous atrophy, body pregnancy, fetal diarrhea, and neoplasia. This tells us that if we can manage inflammation and infection, we are targeting approximately one-third of the problem.⁴ 

The vast majority of pregnancies will require no intervention. For anxious clients or mares at higher risk for problematic pregnancy, routine monitoring can be instituted. Case selection can be based on the following:

• History of previous and potentially repeatable problems during pregnancy
• Signs of potential premature parturition including premature mammary development, premature relaxation of ligaments around the tail head, premature vulvar elongation, vulvar discharge, urine staining beneath the vulva, or discharge found on the ventral aspect of the tail
• Current systemic compromise of the mare including infectious disease, metabolic disease, laminitis, colic
• Abnormalities detected during evaluation before the pregnancy such as cervical lacerations, vesicovaginal urine reflux, poor endometrial biopsy grade, metabolic or other systemic disease
• Abnormalities found during pregnancy monitoring. Monitoring can include physical examination, body condition score assessment, regular ultrasonographic assessment of the combined thickness of the uterus and placenta (CTUP), transabdominal ultrasonographic assessment of the fetus and placenta, hormonal measurements

It is presumed that placentitis observed near the cervical star is due to pathogen ascent via the caudal reproductive tract and subsequently through the cervix. It stands to reason that routine transrectal ultrasonographic monitoring of this region can be helpful in attempting to achieve diagnosis of placentitis sufficiently early to make a difference. Similarly, in women, inflammatory diseases of the genital tract are among the most frequent diseases during pregnancy and are thought to account for 25% to 40% of preterm births, with the most frequent and serious ascending from the lower genital tract.⁵

Renaudin et al (1997) published the evaluation of CTUP of 9 normal mares throughout gestation. Subsequently, evaluation of the CTUP has been incorporated into routine pregnancy monitoring programs (Table 2).⁶
 

In a field application of transrectal ultrasonographic assessment of the CTUP in 477 Thoroughbred mares on one farm in central Kentucky, placentitis was diagnosed in 3.1% of the mares. The abortion rate among mares with placentitis was 15.8%, with pregnancy loss occurring at an average of 62 days (range, 7 to 90 days) after detection and treatment onset. Of the nonaborting placentitis cases, 87% produced live foals, with a mean gestational length of 327 ± 2.23 days. The mean birth weight of live foals from affected mares (48.8 ± 1.56 kg) was not significantly different from foals born from unaffected mares (53.9 ± 0.28 kg). Foals that were born dead or died shortly after birth (not counted among live foals) were significantly smaller than surviving foals.⁷

Transabdominal ultrasonography can also be used to assess fetal health. Parameters that can be included to form a biophysical profile are fetal heart rate, fetal aortic diameter, fetal activity, fetal breathing movements, orbit diameter, tracheal diameter, stomach dimensions, kidney dimensions, gonadal dimensions, fetal fluid depth, uteroplacen- tal thickness, and uteroplacental contact.⁸

Bucca (2011) has described the normal cervical dimensions of the pregnant mare.⁹ In humans, ultrasonographic cervical measurement has proved to be useful to detect patients at risk for preterm delivery regardless of parity or obstetric history.¹⁰

Maternal plasma total progestagen concentrations can be used to predict fetal health. The true concentrations of total progestagens being measured can vary substantially depending on the assay and antibody cross reactivity. Progestagen production involves the fetal adrenal; thus, progestagens can reflect fetal adrenocortical activity and stress. Rapid progestagen decline suggests severe fetal compromise. Progestagens at a level higher than normal are normally seen before spontaneous parturition at term and can also be seen in cases of placentitis or poor placental function. Progestagen levels that fail to normally rise before parturition suggest ergot alkaloid toxicity. In general, mares with high total progestagen concentrations are more likely to deliver live foals than those with low concentrations.¹¹  [...]