At 37 weeks gestation, a pregnancy is considered near-term. Fetal heart rate (FHR) monitoring is a standard part of late-pregnancy care, providing insight into the baby’s well-being within the uterine environment. An elevated FHR, known as fetal tachycardia, requires evaluation by a healthcare team. While this finding can be concerning, most causes are temporary and easily addressed, though a small percentage can signal a serious underlying issue.
Understanding Fetal Heart Rate at 37 Weeks
The normal baseline fetal heart rate in the third trimester, including at 37 weeks, typically ranges between 110 and 160 beats per minute (bpm). This range reflects the healthy balance of the fetal autonomic nervous system, which controls heart function. A rate consistently outside this window prompts closer examination, as it may indicate the baby is responding to a change in its environment.
Fetal tachycardia is medically defined as a sustained heart rate above 160 bpm. Healthcare providers often categorize mild tachycardia as a rate between 161 and 180 bpm, while anything consistently above 180 bpm is considered severe. The heart rate naturally fluctuates with fetal movement and sleep cycles, so a diagnosis of tachycardia is based on a sustained elevation over a period of time, not a momentary spike.
Monitoring the FHR is a way to indirectly assess the baby’s oxygenation and overall health. The heart rate is a sensitive indicator of fetal well-being because the baby’s body responds to stress, such as low oxygen levels, by increasing its heart rate to improve circulation. A sustained high rate, therefore, suggests a persistent stimulus or a primary heart rhythm issue.
Potential Reasons for an Elevated Fetal Heart Rate
Many instances of elevated FHR at 37 weeks are temporary and relate to conditions affecting the mother. The most common cause is maternal fever, which raises the mother’s core body temperature and, subsequently, the baby’s temperature, leading to a faster heart rate. Maternal dehydration can also trigger an elevated FHR as reduced maternal blood volume can transiently affect placental blood flow and oxygen delivery to the baby.
Maternal anxiety or stress causes a release of stress hormones that can cross the placenta and directly stimulate the baby’s heart rate. Certain medications the mother may be taking can also cause this effect, such as sympathomimetic drugs used to treat asthma. These transient causes are often resolved by treating the underlying maternal condition, such as administering intravenous fluids for dehydration or acetaminophen for a fever.
More concerning causes involve a primary problem with the baby’s health or its immediate environment. Fetal infection, such as chorioamnionitis (an infection of the placenta and amniotic fluid), frequently presents with sustained fetal tachycardia as a sign of inflammation and stress. A high heart rate can also be an early sign of fetal hypoxia, indicating the baby is attempting to compensate for a lack of oxygen delivery from the placenta.
Less common, but serious, causes include a fetal arrhythmia, such as Supraventricular Tachycardia (SVT), due to an electrical signaling issue. Maternal conditions like uncontrolled hyperthyroidism can also lead to fetal tachycardia, as the excess thyroid hormone crosses the placenta and stimulates the baby’s metabolism and heart. When the heart beats too fast, it can lead to heart failure and a condition called hydrops, where fluid accumulates in the baby’s body.
Diagnostic Testing and Further Assessment
When sustained fetal tachycardia is detected, the first step in assessment is typically a non-stress test (NST) using an electronic fetal monitor. The NST evaluates the baby’s heart rate pattern, looking for accelerations, which are brief increases in heart rate above the baseline, usually in response to fetal movement. At 37 weeks, a reassuring, or “reactive,” NST is characterized by sufficient accelerations within a defined period.
The pattern of the heart rate variability is also an indicator; a healthy heart tracing shows moderate variability, meaning a slight, constant fluctuation in the baseline rate. The presence of moderate variability and accelerations on the NST, even with a mildly elevated baseline, is reassuring that the baby is currently well-oxygenated. Conversely, sustained tachycardia combined with reduced or absent variability is a non-reassuring finding that suggests the baby may be under stress.
If the initial monitoring is non-reassuring, a more comprehensive Biophysical Profile (BPP) may be performed, which combines the NST with an ultrasound examination. The BPP assigns a score based on five parameters:
- The non-stress test
- Fetal breathing movements
- Body movements
- Muscle tone
- The amount of amniotic fluid
A high score, such as 8 out of 10, suggests the baby is handling the elevated heart rate well, while a low score, typically 4 or less, often prompts immediate intervention.
If an arrhythmia is suspected, a specialized ultrasound called a fetal echocardiogram is necessary to determine the heart rhythm disturbance and to check for any structural heart abnormalities. Maternal blood work is also routinely performed to rule out underlying maternal causes, such as infection or thyroid dysfunction.
Medical Management and Next Steps
Management of fetal tachycardia at 37 weeks depends entirely on the underlying cause and the results of the diagnostic testing. If the elevated FHR is determined to be due to a benign, transient cause, the management focuses on treating the mother. Simple measures like increasing maternal fluid intake and managing fever with appropriate medication often resolve the tachycardia quickly, and the pregnancy can continue with close monitoring.
For cases where a serious underlying condition is confirmed, the action plan shifts toward intervention, especially since 37 weeks is considered term. If chorioamnionitis is suspected, the mother will be started on broad-spectrum antibiotics, and delivery is often recommended to prevent further risk to both the mother and the baby. The baby’s condition is closely monitored throughout this process.
If the elevated FHR is due to a fetal arrhythmia like SVT, and the baby is showing signs of distress or heart failure, transplacental anti-arrhythmic medications may be administered to the mother to control the baby’s heart rhythm. Given the near-term status, if the heart rate remains uncontrolled or the BPP score is low, prompt delivery is often necessary, either via induction or Cesarean section, to allow for direct treatment of the baby. Close collaboration between obstetrics, neonatology, and cardiology teams guides the timing and method of delivery.