Fetal heart rate (FHR) monitoring is a routine practice during the third trimester of pregnancy, providing insight into the baby’s well-being. Fetal tachycardia is defined as a sustained heart rate above 160 beats per minute (bpm). While temporary increases are often benign, a persistently elevated rate can signal an underlying condition requiring medical attention. Understanding the causes of a high FHR is important, as they range from common maternal changes to specific fetal health issues.
Understanding Normal Fetal Heart Rate Fluctuations
The typical baseline FHR for a fetus in the third trimester falls within a range of 110 to 160 bpm. This rate is not static and naturally moves up and down throughout the day. Temporary increases above the baseline, called accelerations, are considered a positive finding. These accelerations often occur in response to fetal movement or external stimulation, indicating a healthy, active fetus with a responsive nervous system.
The fetus experiences sleep-wake cycles, which influence the heart rate pattern. During deep sleep, the FHR may be at the lower end of the normal range, while active wakefulness naturally increases the rate. These transient changes are normal physiological responses. Sustained high readings, however, suggest a continuous stimulus is preventing the rate from returning to its normal baseline.
Maternal Factors That Increase Fetal Heart Rate
Many instances of fetal tachycardia stem from a temporary change in the mother’s systemic state. Maternal fever or infection is the most common cause of sustained mild fetal tachycardia. The mother’s elevated body temperature directly increases the fetal temperature, triggering a compensatory stress response that raises the heart rate. Maternal infections, such as chorioamnionitis, can also lead to fetal tachycardia as the fetus reacts to the inflammatory process.
Maternal dehydration can influence the FHR by reducing the mother’s blood volume. This can lead to slightly lower oxygen levels, prompting the fetus to increase its heart rate to compensate. Maternal hyperthyroidism, an overactive thyroid gland, results in excess thyroid hormones that cross the placenta. These hormones act as a stimulant on the fetal heart, causing a sustained increase in rate.
Certain medications or substances consumed by the mother can also act as stimulants. Medications like some asthma treatments and sympathomimetic drugs can cause a faster FHR. Excessive caffeine intake or the use of illicit stimulant drugs similarly cross the placenta. These substances directly affect the fetal autonomic nervous system, leading to tachycardia.
Fetal Conditions Associated with Tachycardia
When the cause is not maternal, fetal tachycardia often points toward conditions originating within the fetus itself. Fetal infection, especially intrauterine infection, causes the fetus to mount an inflammatory and stress response, resulting in a persistent high heart rate. This sustained tachycardia is a significant marker for conditions like fetal sepsis or chorioamnionitis.
Fetal anemia, a reduced capacity of the blood to carry oxygen, can lead to compensatory tachycardia. The heart works harder and faster to circulate the limited oxygen supply more quickly throughout the body. Fetal cardiac arrhythmias, or abnormal heart rhythms, are another direct cause. Supraventricular tachycardia (SVT) is the most frequent type, where an electrical problem causes the heart chambers to beat excessively fast, often exceeding 200 bpm.
In cases of fetal hypoxia (insufficient oxygen), the initial compensatory mechanism is often an increase in the heart rate. This attempt to improve oxygen delivery can manifest as tachycardia before other signs of fetal distress become apparent. In rare instances, fetal hyperthyroidism, where the thyroid gland is overactive, directly stimulates the heart, causing a rapid rate.
Medical Response and Monitoring
When sustained fetal tachycardia is detected, the first medical response is to identify and address any reversible maternal causes. If maternal fever or dehydration is the cause, treating the infection or administering intravenous fluids may resolve the high FHR. If the cause remains unclear or the rate is severely elevated, specialized monitoring is initiated to assess fetal well-being.
Diagnostic tests may include a Non-Stress Test (NST) to observe the heart rate pattern over time, and a Biophysical Profile (BPP) using ultrasound to evaluate fetal movement, breathing, tone, and amniotic fluid volume. A fetal echocardiogram, a specialized ultrasound of the fetal heart, is often performed to diagnose a primary cardiac arrhythmia or structural issue. For persistent arrhythmias like SVT, medications may be administered to the mother to cross the placenta and slow the FHR. Parents should contact their healthcare provider if they notice a sudden, sustained change in fetal movement.