Monitoring Fetal Heart Rate in Horses

Q. Is it possible to hear a fetal heart rate with a high-quality stethoscope? If so, at about what stage of gestation, and what is the optimal position of the drum? I have a 15-year-old Morgan mare that is currently at 240 days gestation, carrying on the right side, and her body condition is between 6 (moderately fleshy) and 7 (fleshy) on the Henneke body scoring chart. I'm not concerned about the health of the fetus, and the mare seems to be thriving. I'm just curious (and thrilled) about the prospects of this baby.

Cathryn Gunther, Cheshire, Connecticut

A. It would be very difficult or almost impossible to auscultate (listen to) the fetal heart beat through the flank of a pregnant mare. One of the main reasons for this would be that the mare's intestinal sounds would interfere with your ability to hear the fetal heart beat. Also, the large size of the mare (hence, a large amount of space between her skin and the foal) will add to the difficulty of listening to the fetal heart beat with a stethoscope.

There are two ways of monitoring the fetal heart rate:

  • The first method is via ultrasonography. The fetal heart beat can first be seen transrectally (using ultrasound by way of the rectum) as early as 25 days of gestation. As gestation advances and the fetus descends into the abdomen, the fetal heart rate can be assessed via transabdominal ultrasonography (which, via a probe on the mare's belly, provides an image of the developing fetus and allows the veterinarian to examine the structure and function of its heart).
  • A second modality of monitoring the fetal heart rate, later in gestation, is via transcutaneous Doppler monitoring, using a device that straps to the mare's abdomen. This device senses fetal movement and heart beats, and once the mare is fitted with it, the fetus can be monitored for several hours in order to assess heart rate during quiet and active periods.

Fetal heart rate is part of a fetal biophysical profile; this is a series of fetal parameters obtained via transabdominal ultrasonography. This evaluation is usually done when evidence exists that the mare could have problems with her pregnancy.

The advantage of transabdominal ultrasonography is that it can be done at the farm with the latest generation ultrasound machines.

The fetal heart rate should be regular, and it decreases as gestation advances. In advanced gestation (greater than 330 days) the fetal heart rate normally is within the mid-60s to mid-70s range. This could vary depending on the fetal activity (movement).

About the Author

Lucas Pantaleon, DVM, Dipl. ACVIM

Lucas Pantaleon, DVM, Dipl. ACVIM, is an equine internal medicine specialist.

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