What Does Deceleration During Labor Mean?

When a person is in labor, their healthcare provider will use different methods to keep track of the fetal heart rate (FHR). Deceleration occurs when the FHR temporarily slows during labor.

Fetal heart rate monitoring looks at patterns of deceleration and acceleration.

The devices used to monitor fetal heart rate during labor can be internal or external. For example, an external method would be an ultrasound device. An internal option is attached to the fetus's scalp and provides more accurate heart rate readings. 

Changes in fetal heart rate during labor can be normal, but they can also indicate a problem for the fetus or the pregnant person. 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). 

deceleration

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Accelerations vs. Decelerations

Accelerations are short-term increases in fetal heart rate by at least 15 beats per minute (bpm) that last at least 15 seconds. These accelerations occur at different times throughout labor and delivery and are a sign that the fetus has an adequate supply of oxygen.

Accelerations and decelerations can both occur during labor. While accelerations indicate that the fetus is doing OK, decelerations can be a sign of a problem.

Types of Deceleration

There are three types of deceleration that may occur during labor. Each type is categorized by the timing of the deceleration during uterine contractions.

Late Deceleration

Late deceleration is a gradual decrease in fetal heart rate after a uterine contraction. It is caused by a decrease in placental blood flow.

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.

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.

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.

What Causes Deceleration?

The cause of deceleration varies depending on the type. For example, 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

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.

Frequently Asked Questions

What causes late deceleration?

Deceleration has various causes, depending on the type. For example, 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).

What causes early deceleration?

Early decelerations are caused by compression of the fetal head during uterine contractions. This results in vagal response (a response that occurs when the vagus nerve is stimulated). The vagal response causes a drop in the fetal heart rate.

What is a normal fetal 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 bpm to 160 bpm. FHR generally decreases as the pregnancy goes on.

When does fetal heart rate slow down?

The FHR begins to slow by midterm. By about 10 weeks of pregnancy, the FHR beings to slow to approximately twice the normal adult heart rate (which is about 60 to 100 bpm). The FHR can also slow down when the fetus is asleep.

However, a slow FHR at 7 weeks gestation or less is linked with a high risk of fetal death during the first trimester.

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