Fetal heart rate (FHR) monitoring is a standard practice throughout pregnancy and labor, providing a window into the developing baby’s condition. Healthcare providers listen to or measure the FHR during routine checkups to ensure the baby is adapting appropriately to the intrauterine environment. While a consistent, steady rate is typically expected, variations are a normal part of fetal life, often resulting from activity or minor external stimuli. An elevated heart rate reading should always be discussed with a medical professional, as only they can accurately interpret the finding within the context of the entire pregnancy. Understanding the definitions and potential causes of an increased heart rate can help people remain informed and prepared for any next steps.
Defining Fetal Heart Rate Tachycardia
The normal range for a fetal heart rate varies slightly by gestational age, but generally settles into a baseline of 110 to 160 beats per minute (bpm) during the second and third trimesters. In early pregnancy, the rate can be higher, peaking around 170 bpm near ten weeks. Fetal tachycardia is formally defined as a sustained heart rate consistently above the upper limit of the normal range. Many clinicians define significant tachycardia as a baseline heart rate greater than 180 bpm lasting for a significant period.
The type of tachycardia matters for assessment. Sinus Tachycardia is a common form where the heart beats quickly but maintains a normal rhythm, usually between 180 and 200 bpm, often in response to external stressors. When the rate exceeds 200 to 220 bpm, it often falls into the category of a tachyarrhythmia, indicating a problem with the heart’s electrical conduction system. Supraventricular Tachycardia (SVT) is a more common type of true tachyarrhythmia involving a rapid rate exceeding 200 bpm due to an electrical malfunction within the heart’s upper chambers.
Common Causes of an Elevated Fetal Heart Rate
An elevated fetal heart rate can be triggered by a variety of factors, many of which originate with the mother’s condition. Maternal fever or infection, such as an intrauterine infection called chorioamnionitis, is a frequent cause because the elevated body temperature directly affects the baby’s temperature and metabolism. Maternal dehydration can reduce blood volume and oxygen delivery, prompting the fetal heart to beat faster as a compensatory mechanism to maintain adequate circulation.
Certain maternal conditions and medications also affect the fetal heart. Maternal hyperthyroidism results in an excess of thyroid hormones that can cross the placenta and increase the baby’s heart rate. Similarly, sympathomimetic drugs, including some decongestants or asthma treatments, can act as stimulants that increase both the mother’s and the baby’s heart rates. Maternal emotional states, such as severe anxiety, can also lead to the release of stress hormones that may temporarily elevate the FHR.
Causes can also arise from the baby’s own physiological state. Anemia in the baby, which reduces the oxygen-carrying capacity of the blood, prompts the heart to pump faster to circulate the available oxygen more quickly. Fetal hypoxia, or a lack of sufficient oxygen, can also result in tachycardia as the baby attempts to compensate for the reduced supply. The elevated heart rate can also be a simple and transient response to fetal activity or stimulation. Less commonly, the cause is an intrinsic issue with the fetal heart itself, such as a structural anomaly or an electrical conduction defect leading to a sustained tachyarrhythmia.
Clinical Assessment and Next Steps
When an elevated fetal heart rate is noted, the first step involves a thorough clinical assessment to determine the underlying cause and the rate’s character. Healthcare providers distinguish between transient tachycardia, which is brief and often benign, and sustained tachycardia, which is persistent and more concerning. A detailed maternal history is taken to look for recent illnesses, fever, drug use, or signs of dehydration that might explain the finding.
Further monitoring tools are used to analyze the heart rate pattern in depth, including specialized ultrasound techniques like M-mode Doppler to visualize the atrial and ventricular contractions. The assessment also focuses on the presence of beat-to-beat variability, which is a sign of a healthy, well-oxygenated fetal nervous system. A specialized ultrasound known as a fetal echocardiogram is often performed to closely examine the heart’s structure and function. This assessment identifies structural heart defects and checks for complications like hydrops fetalis, which is fluid accumulation signaling heart failure due to a prolonged, fast rate.
If the tachycardia is transient or related to an easily correctable maternal factor like dehydration, the baby is typically monitored closely. If the high rate is sustained, is associated with decreased variability, or is caused by a true tachyarrhythmia, treatment may be initiated. This often involves administering antiarrhythmic medication to the mother, which then crosses the placenta to slow the baby’s heart rate. The goal is to convert the rhythm to a normal rate, prevent complications, and ideally allow the pregnancy to progress to a safe delivery near term.