Eye on the prize: fetal monitoring in late gestation

Monitoring of the health of the equine fetus is based upon ultrasonographic evaluation, observation of the mare, and clinical intuition.

Transabdominal and transrectal ultrasonography allow assessment of CTUP, fetal fluids and fetal viability.  Renaudin et al. published the evaluation of the combined thickness of the uterus and the placenta (CTUP) of 9 normal mares throughout gestation.¹  During field application of transrectal ultrasonographic assessment of the CTUP in 477 Thoroughbred mares, placentitis was diagnosed in 3.1%.  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 non-aborting 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).  Normal cervical dimensions of the pregnant mare have been described.²  More on placental evaluation another time.  Widely practiced and very important.

Transabdominal ultrasonography can also be used to form a biophysical profile, assessing fetal heart rate, fetal aortic diameter, fetal activity, fetal breathing movements, orbit diameter, tracheal diameter, stomach dimensions, kidney dimensions, gonadal dimensions, fetal fluid depth, uteroplacental thickness, and uteroplacental contact.³

Hormonal monitoring is widely performed, controversial in some circles, and incompletely understood.  More information is becoming available all the time so watch this space.  Maternal plasma total progestagen concentrations can be used to predict fetal health.  Progestagen production involves the fetal adrenal, reflecting 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 prior to parturition suggest ergot alkaloid toxicity.  Mares with high total progestagen concentrations are more likely to deliver live foals than those with low concentrations.⁴  Estrogen concentrations in isolation appear less useful than progestagens for predicting fetal health.  Estrone sulfate is the most frequently measured.  High estrone sulfate levels (>100 ng/mL) indicate a viable fetus.  Low estrone sulfate levels (<10 ng/mL) indicate fetal loss or a non-pregnant mare.  Estrone sulfate may be transiently decreased with compromised pregnancy.⁴  A recent study showed no changes in estrone sulfate concentrations in experimentally-induced placentitis prior to fetal demise.⁵  Estradiol 17beta has been shown useful to monitor treatment response in mares with experimentally induced placentitis.⁶  Utility in spontaneously occurring field placentitis is not yet established.  The placenta is the sole source of circulating relaxin in the mare, therefore it may be a biochemical marker of placental function, fetal health, and a predictor of pregnancy outcome.  A commercially available relaxin assay, if one becomes available, may prove to be useful to assess loss in placental function.⁷

Inflammatory markers have also received close attention.  These would provide an easy way to detect placentitis and monitor response to treatment.  Complete blood counts, fibrinogen and serum amyloid A (SAA) levels are commonly measured in field placentitis cases.  As SAA is a sensitive marker of inflammation all other possible pathologies must be ruled out.  While some value was discovered monitoring experimentally-induced placentitis⁸, field utility in spontaneous cases has not been established.

Cervical or uterine cultures of inflammatory discharges from pregnant mares are valuable if inflaming the vulvar, vestibulovaginal and cervical seals can be avoided. 

Where the need for intervention to improve the chances of a positive pregnancy outcome is determined, supportive measure include control of the agent(s) precipitating the compromise, management of inflammation, and hormonal support of the fetoplacental unit.  I will talk more about those over the next few weeks.

 

1.    Renaudin C, Troedsson MHT, Gillis C et al. Ultrasonographic evaluation of the equine placenta by transrectal and transabdominal approach in pregnant mares. Theriogenology 1997;47:559-573.

2.    Bucca S and Fogarty U. Ultrasonographic cervical parameters throughout gestation in the mare. American Association of Equine Practitioners 2011;57:235-241.

3.    Bucca S, Fogarty U, Collins A et al. Assessment of feto-placental well-being in the mare from mid-gestation to term: Transrectal and transabdominal ultrasonographic features. Theriogenology 2005;64:542-557.

4.    Ousey JC. Hormone profiles and treatments in the late pregnant mare. Vet Clin North Am: Equine Practice 2006;22:727-747.

5.    Canisso IF, Ball BA, Esteller-Vico A et al. Changes in maternal androgens and oestrogens in mares with experimentally-induced ascending placentitis. Equine Vet J 2017;49:244-249.

6.    Canisso IF, Curcio BR, Burden C et al. Peripheral Markers Profiles in Response to Treatment for Experimental Placentitis in Mares. J Equine Vet Sci 2018;66:240-.

7.    Ryan PL, Christiansen DL, Hopper RM et al. Evaluation of systemic relaxin blood profiles in horses as a means of assessing placental function in high-risk pregnancies and responsiveness to therapeutic strategies. Ann N Y Acad Sci 2009;1160:169-178.

8.    Coutinho da Silva MA, Canisso IF, Macpherson ML et al. Serum amyloid A concentration in healthy periparturient mares and mares with ascending placentitis. Equine Vet J 2013;45:619-624.