Key Takeaways
- Throughout your pregnancy and during labor, your healthcare provider will measure your baby’s heart rate to look out for any potential problems.
- During labor, accelerations or increases in heart rate are a sign that your baby is doing well and has adequate oxygen.
- Late decelerations or decreases in heart rate during labor can mean less blood flow from the placenta.
Fetal heart rate changes, such as accelerations and decelerations, are vital indicators of a baby’s well-being during late pregnancy and labor. An abnormal rate or specific deceleration patterns might suggest the need for urgent medical attention to ensure the baby receives adequate oxygen.
What’s a Normal Heart Rate?
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 beats per minute (bpm) to 160 bpm. FHR generally decreases as the pregnancy goes on.
How Do Fetal Heart Rate Accelerations Work?
Accelerations are brief increases in fetal heart rate by at least 15 beats per minute that last at least 15 seconds.
In the third trimester, your healthcare provider might perform a non-stress test. This non-invasive test measures your baby’s heart rate during movement. A normal increase, called a “reactive” or “reassuring” result, indicates everything is fine. If the heart rate doesn’t increase, it might signal a problem.
During labor, your healthcare provider will use various methods to monitor the fetal heart rate. Accelerations throughout labor and delivery indicate that the fetus has enough oxygen.
What Causes Fetal Heart Rate Decelerations?
Deceleration happens when the fetal heart rate temporarily slows during labor. There are three types: early, late, and variable. Early decelerations are usually normal, but late and variable ones may indicate a problem.
Early Deceleration
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:
- A decrease in blood flow to the brain
- A low heart rate
- Low blood pressure
Late Deceleration
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 acidemia), 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.
Variable Deceleration
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.
When Does the Fetal Heart Rate Slow Down?
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.
What to Expect From Fetal Heart Monitoring
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.
The Benefits and Risks of Fetal Monitoring
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:
- Reassuring: No potential problems are detected.
- Nonreassuring: This term is often used in place of “fetal distress” and suggests that there is a problem, such as a fetus not getting enough oxygen.
- Ominous: Patterns of readings (such as late decelerations) that indicate emergency intervention is required.
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.
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