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During your last trimester of pregnancy and throughout labor, your baby's heart rate and rhythm will be monitored regularly to make sure all is well. During this time, the fetal heart rate should be between 110 and 160 beats per minute. An abnormal heart rate could mean there is a problem; for example, your baby might not be getting enough oxygen.
Changes in fetal heart rate during late pregnancy or labor can be normal, but they can also indicate a problem for the fetus or the mother. Specific patterns of deceleration, such as late deceleration, can be signs of fetal distress, which may require emergency interventions, such as a Cesarean section delivery (C-section).
This article discusses fetal heart rate decelerations and accelerations during late pregnancy or labor. It also goes over how healthcare providers monitor your baby's heart rate and what might happen if concerning changes are detected.
Fetal heart rate (FHR) can be detected from approximately 6 weeks gestation. There is some variation in what experts consider the "normal" baseline for FHR, but it's generally accepted to range from 110 bpm to 160 bpm. FHR generally decreases as the pregnancy goes on.
Accelerations are short-term increases in fetal heart rate by at least 15 beats per minute (bpm) that last at least 15 seconds.
During the third trimester of pregnancy, your healthcare provider may do a non-stress test. This is a non-invasive test that measures your baby's heart rate during times of movement. If your baby's heart rate increases as expected, it is considered a "reactive" or "reassuring" result. If it does not, this could be a sign of a problem.
When you are in labor, your healthcare provider will use different methods to keep track of the fetal heart rate. Accelerations may occur at different times throughout labor and delivery and are a sign that the fetus has an adequate supply of oxygen.
Deceleration occurs when the FHR temporarily slows during labor. Three types of deceleration may occur: early, late, and variable. While early decelerations are usually normal, late and variable decelerations can point to a problem.
Early deceleration describes the symmetrical decreases and return-to-normal of the fetal heart rate that is linked to uterine contractions. The decrease in heart rate occurs gradually.
With early decelerations, the point when the fetal heart rate is at its slowest (called the nadir ) and the peak of the uterine contraction happen at the same time. Therefore, the fetal heart rate is recorded at its lowest rate for 30 seconds (or longer) at the beginning of the early deceleration cycle.
Early decelerations are caused by the compression of the baby’s head during uterine contractions. The compression causes vagal stimulation, which slows the fetal heart rate.
The vagal response occurs when there is stimulation of the vagus nerve. The stimulation can lead to physical symptoms in the body, such as:
Late decelerations (a drop in the fetal heart rate after uterine contractions) are caused by a decrease in the placental blood flow. This results in insufficient oxygen supply to the fetus (uteroplacental insufficiency).
Late decelerations may indicate that a fetus has high levels of acid in the blood (a condition called impending fetal academia), which is often caused by a lack of oxygen.
Usually, the onset of low fetal heart rate, the nadir, and the recovery coincide with the contraction’s onset, peak, and end.
A variable deceleration is a very quick decrease in fetal heart rate of 15 bpm or more, that lasts at least 15 seconds (but may last up to two minutes) before the heart rate returns to baseline.
The onset of fetal slow heart rate, as well as the duration of the decelerations, varies with uterine contractions.
Variable decelerations can be a sign that the umbilical cord is compressed. When this happens, the baby may not be getting enough oxygen or other nutrients. A baby that is experiencing multiple variable decelerations may need to be delivered by Caesarian section.
Between the fifth and seventh week of pregnancy, the fetal heart rate is typically between 90 and 110 beats per minute. Between weeks 8 and 12, the heart rate speeds up to between 140 and 170 beats per minute. By week 12, it starts to slow down and usually remains at around 110 to 160 beats per minute until the 26th week. It may slow down a little more during the last 10 weeks of pregnancy.
The devices used to monitor fetal heart rate during labor can be internal or external.
External fetal monitoring may be done with a Doppler ultrasound device. This is the same device used to monitor your baby during prenatal visits. Continuous monitoring can also be done externally with an ultrasound probe attached to your abdomen. With this method, the baby's heart rate is shown on a computer screen and printed on paper so your healthcare provider can monitor it.
Internal monitoring may be done if your healthcare provider isn't getting a good reading from external methods. It can only be done if the amniotic sac has already broken. This method involves placing an electrode on your baby's scalp. The electrode is connected to a wire that runs through your cervix. Internal monitoring may be uncomfortable, but it provides more accurate heart rate readings.
Electronic fetal heart rate monitoring can be used to evaluate the overall health of the fetus during labor. The primary benefit of monitoring is early detection of fetal distress, but there is also a risk of false-positive interpretations of the readings (also called tracings).
The fetal heart rate undergoes continuous stimuli adjustments (such as from contractions). The outcome of the readings are categorized in one of three ways:
However, the inconsistent interpretation of the FHR readings can cause false-positive ominous readings, which could lead to unnecessary emergency interventions, such as a C-section.
Fetal heart rate accelerations and decelerations can tell your healthcare provider how your baby is doing and whether or not there are any potential problems.
Your healthcare provider will measure your baby's heart rate throughout pregnancy. During late pregnancy, they may do a "non-stress" test to make sure your baby's heart rate increases as expected during movement. During labor, your baby's heart rate will be monitored continuously.
Accelerations or increases in heart rate are a sign that your baby is doing well. Early decelerations happen during contractions and are also considered normal. However, late decelerations may indicate there is a decrease in blood flow from the placenta. A variable deceleration can happen when the umbilical cord is temporarily compressed and can be a sign that the baby isn't getting enough oxygen.
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By Sherry Christiansen
Christiansen is a medical writer with a healthcare background. She has worked in the hospital setting and collaborated on Alzheimer's research.
