Study guide
3.1 Accelerations
Certified in Electronic Fetal Monitoring C-EFM
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1 Exam Foundations and Fetal Monitoring
1.1 About the C-EFM Exam 1.2 Physiology of Fetal Oxygenation 1.3 Monitoring Instrumentation and Modes
2 FHR Baseline and Variability
2.1 Baseline Fetal Heart Rate 2.2 Baseline Variability 2.3 Fetal Tachycardia and Bradycardia
3 Accelerations and Decelerations
3.1 Accelerations 3.2 Early Decelerations 3.3 Late Decelerations 3.4 Variable and Prolonged Decelerations
4 Pattern Interpretation and Acid-Base
1.1 The NICHD Three-Tier Category System 1.2 Special Patterns: Sinusoidal, Pseudosinusoidal, and Arrhythmias 1.3 Fetal Acid-Base Assessment and Cord Gases
5 Uterine Activity and Antepartum Testing
5.1 Uterine Activity 5.2 Antepartum Testing: NST and CST 5.3 Biophysical Profile and Doppler 5.4 Maternal-Fetal Conditions Affecting the FHR
6 Clinical Management and Exam Strategy
5.1 Intrauterine Resuscitation 5.2 Operative and Escalation Decisions 5.3 Documentation, Communication, and Legal Issues 5.4 Exam-Day Strategy
3.1 Accelerations
Key Takeaways Explain This Topic AI
An acceleration is a visually apparent ABRUPT increase in fetal heart rate (FHR) with onset to peak in under 30 seconds
At 32 weeks or more, the rise must be 15 bpm or more above baseline lasting 15 seconds or longer (the '15 by 15' rule)
Before 32 weeks, the threshold relaxes to 10 bpm or more above baseline lasting 10 seconds or longer ('10 by 10')
Accelerations reliably predict the absence of fetal metabolic acidemia at that moment and are the basis of NST reactivity
A prolonged acceleration lasts 2 minutes or more but less than 10 minutes; 10 minutes or more is a baseline change
Last updated: June 2026
What the 2008 NICHD Definition Requires
The National Institute of Child Health and Human Development (NICHD) 2008 nomenclature defines an acceleration as a visually apparent abrupt increase in the fetal heart rate (FHR) above the baseline. The word abrupt carries a precise, testable meaning: the time from the onset of the rise to the peak is less than 30 seconds . That sharp onset is exactly what separates a true acceleration from a slow drift in the baseline rate, and the C-EFM exam expects you to apply the 30-second rule rather than rely on how the strip "looks."
Two measurements define a qualifying acceleration, and both are anchored to gestational age . At 32 weeks of gestation or more , the peak must be 15 bpm or more above the baseline , and the acceleration must last 15 seconds or longer — measured from the onset back to the return to baseline. This is the familiar "15 by 15" rule. Before 32 weeks , the immature fetal autonomic nervous system generates smaller, briefer accelerations, so NICHD lowers the bar to 10 bpm or more for 10 seconds or longer ("10 by 10").
Acceleration Thresholds by Gestational Age
- Parameter 32 weeks or more (term) Less than 32 weeks (preterm)
- Amplitude above baseline 15 bpm or more 10 bpm or more
- Duration (onset to return) 15 sec or more 10 sec or more
- Onset to peak (shape) Less than 30 sec (abrupt) Less than 30 sec (abrupt)
- Upper duration limit Less than 2 min Less than 2 min
- Why Accelerations Reassure: The Physiology
Accelerations are episodic or periodic events driven by a brief surge in sympathetic tone, usually in response to fetal movement, scalp stimulation, or vibroacoustic stimulation. Producing an acceleration requires an intact, well-oxygenated fetal central nervous system (CNS) that can mount an autonomic response. That single fact is the engine behind every clinical use of accelerations on this exam: a fetus that is acidemic and centrally depressed cannot generate one. Therefore the presence of an acceleration reliably predicts the absence of fetal metabolic acidemia at the moment it occurs.
The converse is not true — the absence of accelerations does not prove acidemia, because a sleeping or sedated fetus may simply not be moving. This asymmetry (present = reassuring; absent = uncertain) is a classic distractor on the C-EFM exam.
Basis of NST Reactivity
The nonstress test (NST) is built entirely on this principle. A reactive NST at term is conventionally defined as two or more accelerations meeting the 15-by-15 criteria within a 20-minute window . A reactive NST is reassuring and predicts good short-term fetal status. Accelerations are also one of the criteria that help define a Category I tracing, because a tracing with present accelerations and moderate variability is essentially never associated with ongoing acidemia.
Scalp-Stimulation-Induced Accelerations
When a tracing is indeterminate — for example, a Category II strip with minimal variability and no spontaneous accelerations — the clinician can provoke one. Digital scalp stimulation (firmly stroking the fetal scalp during a vaginal exam) or vibroacoustic stimulation (VAS) applied to the maternal abdomen prompts the fetus to respond.
A provoked acceleration that meets the 15-by-15 threshold carries the same meaning as a spontaneous one: it predicts the absence of metabolic acidemia and can substitute for the far more invasive fetal scalp blood pH sampling . This is one of the highest-yield management facts on the exam.
Prolonged Acceleration vs. Baseline Change
A prolonged acceleration lasts 2 minutes or more but less than 10 minutes . Duration is decisive: once an elevation persists for 10 minutes or longer , NICHD no longer calls it an acceleration at all — it is reclassified as a change in the baseline rate . Many candidates miss this boundary, so commit the 2-minute floor and 10-minute ceiling to memory.
Worked example: A 39-week fetus is on the monitor. During a 20-minute NST, the nurse counts two FHR rises: the first peaks 17 bpm above baseline within 12 seconds of onset and lasts 18 seconds; the second peaks 16 bpm above baseline within 10 seconds of onset and lasts 22 seconds. Both are abrupt (onset to peak under 30 sec) and both meet 15-by-15 at term, so each is a qualifying acceleration. Two such accelerations in 20 minutes meet the criteria for a reactive NST — a reassuring result that predicts the absence of metabolic acidemia, with no intervention required.