Fetal Monitoring Basics: Baseline, Variability, Accelerations, Decelerations
If you're heading into OB/GYN residency or currently rotating on labor and delivery, you’ve likely seen a lot of fetal monitoring—but how much of it do you actually understand?
In this episode, I’ll walk you through the basics of electronic fetal monitoring (EFM), what we’re really trying to assess, and how to interpret the different patterns you'll see on the fetal heart rate tracing.
But first—if you just matched into OB/GYN, congratulations! I created a free resource just for you: the OB/GYN Residency Starter Pack: A 5-Step Guide for Incoming Interns. It’s designed to help you feel more prepared and less overwhelmed as you get ready to start residency. You can download it through the show notes or head over to my Instagram and comment “intern” on any post for an instant link.
Now, let’s dive in.
Why We Use Electronic Fetal Monitoring
The fetal brain controls heart rate through a combination of sympathetic and parasympathetic responses, so the heart rate is a useful indicator of fetal oxygenation. The main goal of EFM is to detect and intervene on fetal hypoxia before it causes harm.
Yes, labor itself causes transient drops in oxygenation—that’s normal. The concern is prolonged hypoxia, which can lead to acidemia, morbidity, or even mortality.
Interestingly, multiple studies have shown that continuous monitoring doesn't significantly improve perinatal morbidity or mortality compared to intermittent monitoring. In fact, continuous monitoring has been associated with increased rates of cesarean and operative delivery.
So why are we still doing this?
One major review of 12 randomized controlled trials (37,000 patients) did show a reduction in neonatal seizures in babies who had continuous monitoring. Also, certain EFM patterns have been associated with neonatal morbidity—like tachycardia, marked variability, and prolonged decelerations.
Most importantly, many of the clinical trials excluded high-risk pregnancies, which make up a significant portion of most labor units. So while intermittent monitoring is totally appropriate for uncomplicated, low-risk patients, continuous monitoring remains the standard for most laboring patients in the U.S.
The 4 Main Components of Fetal Heart Tracings
When someone asks you to read a fetal heart rate strip, these are the four components you need to report:
Baseline
Variability
Accelerations
Decelerations
Let’s break them down.
1. Baseline
The baseline is the mean fetal heart rate rounded to increments of 5 bpm. A normal baseline is 110–160 bpm.
Less than 110 = Bradycardia
Greater than 160 = Tachycardia
Pro tip: When estimating the baseline on the tracing, imagine placing a straight ruler along the “base” of the squiggles. That’s your baseline.
2. Variability
Variability refers to the amplitude fluctuations in the baseline (and contribute to the squiggly appearance)
Absent: No detectable variability (think flatline)
Minimal: <5 bpm amplitude (think almost flatline, with minimal squiggles)
Moderate: 6–25 bpm amplitude (this is normal)
Marked: >25 bpm amplitude (think chaotic, scribbly line)
3. Accelerations
Accelerations are abrupt increases in fetal heart rate, where the onset to peak is less than 30 seconds.
Increase is by >15 bpm and lasting >15 seconds
≥32 weeks gestation: At least 15 bpm above baseline for at least 15 seconds (15x15)
<32 weeks gestation: At least 10 bpm above baseline for at least 10 seconds (10x10)
If it lasts longer than 2 minutes, it’s a prolonged acceleration
If it lasts longer than 10 minutes, it’s no longer an acceleration—it’s a baseline change.
4. Decelerations
This is where it gets tricky—and where exam questions often focus. Decelerations are classified by timing, shape, and cause.
Early Decelerations
Caused by fetal head compression
Gradual decrease, with onset to nadir ≥30 seconds
Symmetric and mirrors the contraction
This is normal - no interventions needed
Late Decelerations
Caused by uteroplacental insufficiency
Gradual decrease, onset to nadir ≥30 seconds
Nadir occurs after the peak of the contraction
Associated with hypoxia
Variable Decelerations
Caused by cord compression
Abrupt decrease (onset to nadir <30 seconds)
Decrease is by >15 bpm and lasting >15 seconds
Can occur before, during, or after contractions
Often look like sharp, jagged drops in heart rate
Prolonged Decelerations
A decrease in fetal heart rate by ≥15 bpm lasting 2 to 10 minutes
If it lasts ≥10 minutes, it's considered a baseline change
If the fetal heart rate remains in the bradycardic range for too long—specifically for 10 minutes or greater—it’s considered terminal bradycardia, which often signals the need for an emergency delivery.
Sinusoidal Pattern
Rare and abnormal, the sinusoidal pattern looks like smooth, regular waves (3–5 per minute) lasting ≥20 minutes. You likely won’t see this often, but it’s important for exams.
Quick Recap
Here’s what you should remember:
Baseline: 110–160 bpm, rounded to the nearest 5 bpm
Variability: Moderate (6–25 bpm) is normal
Accelerations: 15x15 or 10x10, depending on gestational age
Decelerations:
Early = head compression
Late = uteroplacental insufficiency
Variable = cord compression
Prolonged = ≥15 bpm drop for ≥2 minutes
Sinusoidal: Smooth, undulating wave pattern, abnormal, rare
Understanding fetal monitoring takes practice, but learning these foundational concepts will help you feel more confident during labor and delivery rotations—and ultimately as a new OB/GYN.
See you next week for another episode!
