Fetal Heart Rate Measurements Aid Health Monitoring

Fetal heart rate monitoring during the second half of pregnancy can help veterinarians easily and reliably assess the health of the unborn foal, according to new research by German and Austrian veterinary scientists. However, it's unlikely to give clues about when a mare will foal.

Fetomaternal electrocardiogram (ECG) equipment, with filtering to enhance the fetal heart recordings, can consistently detect the cardiac activity of unborn foals from day 173 (of 340 gestational days) onwards, reported Christina Nagel, MSc, PhD candidate at the Graf Lehndorff Institute for Equine Science in Neustadt (Germany) and primary researcher in the study. In the scope of her study, the fetal heart rate could almost always be found within the first ten minutes.

"Fetal ECG is an important tool, in addition to transrectal or transabdominal ultrasounds, for monitoring high-risk pregnancies and assessing their potential outcomes," said Christine Aurich, DVM, PhD, professor at the Graf Lehndorff Institute and at the University of Veterinary Sciences in Vienna and co-author of the study.

It can also be used to determine whether a fetus has died, which is difficult to determine via ultrasound alone, she added.

Electrode placement for fetal heart rate montioring

Placement of the electrodes to measure fetal heart rate varies depending on the mare’s stage of gestation.

Nagel's team determined that fetal heart beats at mid-term averaged 110 beats per minute and slowly dropped to around 80 beats per minute by the end of pregnancy. In healthy fetuses, the average rates remained consistent morning and evening and reflected no changes that could suggest impending labor in the days prior to foaling. However, 80% of the fetuses did show a mild increase in heart rate approximately thirty minutes before birth, she reported.

Slight rate changes from beat to beat, called heart rate variability (HRV), occurred throughout the second half of gestation. As in humans, this is a sign of a healthy, normal fetus and is often related to fetal activity. In the study, HRV increased in the last two months before birth, probably the result of a maturing autonomous nervous system, Nagel said. However, this increase is late compared to human fetuses, and might be associated with the fact that horses mature much later in utero than most other species.

Interestingly, even though fetal heart rate tended to increase during foaling, HRV did not decrease as it does during birth in most other species, Nagel stated. As HRV is frequently associated with stress levels, this could suggest that the birthing process is relatively unstressful for foals, she reported.

"ECG equipment is fairly inexpensive," said Aurich, "but it's really an invaluable tool to any practitioner working regularly with broodmares."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.