A fetal acceleration refers to a temporary increase in a baby’s heart rate above its usual resting pace, known as the baseline. This change is an abrupt rise in the fetal heart rate, with the increase typically lasting less than 2 minutes. The heart rate then returns to its previous baseline level. This elevation in heart rate is observed during various assessments of fetal health.
The Significance of Fetal Accelerations
Fetal accelerations serve as an indication of a baby’s well-being and a healthy, responsive nervous system. These increases in heart rate occur in response to fetal movement, suggesting that the baby is receiving adequate oxygen. The presence of accelerations signals that the fetus is not experiencing acidemia, a condition related to insufficient oxygen.
A “reactive” heart rate pattern, observed during monitoring, signifies fetal health. For a baby at or beyond 32 weeks of gestation, a reactive pattern is defined by two or more accelerations within a 20-minute period. Each acceleration should show an increase of at least 15 beats per minute (bpm) above the baseline, lasting for a minimum of 15 seconds. For babies under 32 weeks, the criteria are often an increase of at least 10 bpm for at least 10 seconds.
How Fetal Accelerations Are Monitored
Fetal accelerations are monitored using a Non-Stress Test (NST), a non-invasive procedure performed in the third trimester. During an NST, the expecting parent reclines in a chair or on an exam table. Two elastic belts are placed around the abdomen.
One belt holds a sensor that monitors the baby’s heart rate, which can be heard. The other sensor tracks uterine contractions, even those that may not be felt. This information is displayed on a monitor or printed onto a paper strip, allowing healthcare providers to observe heart rate patterns over a period, usually 20 to 30 minutes.
Factors Influencing Fetal Accelerations
Factors can influence whether fetal accelerations are observed, and their temporary absence does not always signal a problem. A primary reason for a lack of accelerations is the baby’s sleep cycle. Fetal sleep states can lead to periods of inactivity, during which accelerations may be less frequent or absent.
The gestational age of the baby also plays a role, as acceleration criteria can vary depending on how far along the pregnancy is. Maternal factors, such as certain medications (including narcotics) or low blood sugar, can also temporarily affect fetal activity and the presence of accelerations.
Responding to Absent Accelerations
If accelerations are not observed during the initial Non-Stress Test, the first step is often to extend the test time. This allows for a longer observation period, up to 40 minutes, to account for the baby’s sleep cycles and allow them to become more active.
To encourage the baby to wake up and move, healthcare providers may use stimulation techniques. This can involve the mother drinking something cold, like juice, or applying a vibroacoustic stimulator to the abdomen. This device emits a buzzing sound to rouse the baby. If accelerations remain absent after these measures, further evaluation is recommended, such as a biophysical profile (BPP). This ultrasound-based assessment provides a comprehensive picture of the baby’s well-being by looking at movement, breathing, and amniotic fluid volume.