Fetal heart rate (FHR) is the measurement of a baby’s heart beats per minute while still in the womb. This rate indicates fetal well-being throughout gestation, offering insight into the baby’s oxygen supply and health. Monitoring FHR is a routine part of prenatal care, checked at every visit to ensure it falls within a healthy range. Understanding expected changes is important, especially in the third trimester, which marks significant physiological maturation.
The Normal Fetal Heart Rate Trajectory
The fetal heart rate decreases and stabilizes as pregnancy progresses toward the third trimester. Early in development, the heart rate starts near the mother’s rate, around 80 to 85 beats per minute (bpm) at five weeks gestation. The rate rapidly accelerates, peaking at approximately 170 bpm around the ninth or tenth week of pregnancy.
Following this peak, the FHR begins a gradual decline, stabilizing into the normal baseline range of 110 to 160 bpm throughout the second and third trimesters. The average FHR continues a slight decrease even within the third trimester, dropping from 141 bpm at 28 weeks to approximately 133 bpm by 40 weeks.
Fetuses closer to term tend to have a heart rate at the lower end of the normal range. This shift is a normal sign of maturation, reflecting a healthy progression toward the rate expected at birth.
Physiological Basis for FHR Changes
The slowing and stabilization of the fetal heart rate is directly linked to the maturation of the autonomic nervous system (ANS). The ANS regulates involuntary body functions, including heart rhythm, and is divided into sympathetic and parasympathetic branches. The sympathetic nervous system acts as the accelerator, increasing the heart rate, and dominates during early pregnancy.
As the fetus grows, the parasympathetic nervous system, specifically the vagus nerve, matures and gains influence. This system acts as the brake, exerting an inhibitory influence that slows the heart rate and establishes the lower baseline rate seen in the third trimester. This shift to parasympathetic control is responsible for the rate decrease.
The interaction between the two ANS branches establishes beat-to-beat variability, the slight fluctuation in heart rate. The central nervous system governs this interaction. Increasing regulation throughout the third trimester is a positive sign of the fetus’s neurological maturity.
Monitoring FHR in the Third Trimester
Clinical monitoring of the fetal heart rate in the third trimester goes beyond measuring the baseline rate (110 to 160 bpm). Providers assess variability and accelerations to gauge the baby’s health status. These assessments are commonly performed through tests like the Non-Stress Test (NST) and the Biophysical Profile (BPP).
The Non-Stress Test uses external sensors on the mother’s abdomen to monitor FHR and uterine contractions over 20 to 40 minutes. The test is considered “reactive” if the fetus shows accelerations, which are brief increases in heart rate of at least 15 bpm lasting 15 seconds or more. These accelerations indicate a healthy, oxygenated nervous system responding to movement.
The Biophysical Profile is a comprehensive assessment combining the NST with an ultrasound evaluation. The ultrasound component scores the fetus on four additional parameters:
- Body movements
- Muscle tone
- Breathing movements
- Amniotic fluid amount
The BPP provides a cumulative score out of 10, with a score of 8 or 10 considered normal and reassuring of fetal well-being.
Recognizing Abnormal Fetal Heart Rate Patterns
Deviations from the normal third-trimester FHR range can signal concerns. Sustained bradycardia, a baseline rate below 110 bpm for 10 minutes or longer, can indicate an issue with fetal oxygen supply. Conversely, sustained tachycardia, a rate consistently above 160 bpm, may be a sign of maternal fever, infection, or fetal distress.
Another concerning pattern involves decelerations, which are temporary drops in the heart rate during uterine contractions or in response to external factors. Decelerations are classified by their appearance and timing relative to contractions. Late decelerations, where the FHR drop begins after the contraction starts, are concerning because they suggest uteroplacental insufficiency.
The absence or significant reduction of beat-to-beat variability (minimal or absent variability) is also a sign of concern. Because variability is controlled by the central nervous system, its loss can indicate that the fetus is compromised, often due to a lack of oxygen or certain medications. Clinicians use abnormal patterns to determine whether immediate intervention is necessary.