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How to Read a CTG (Cardiotocography)

A CTG trace has two channels:

Upper channel - Fetal Heart Rate (FHR) in beats per minute (bpm)
Lower channel - Uterine contractions (tocogram)

The CTG Strip - What You're Looking At

Here is a normal CTG: baseline ~130 bpm, good variability, no decelerations, regular contractions below.

Normal CTG - Creasy & Resnik's Maternal-Fetal Medicine

Normal FHR pattern: ~130 bpm, normal variability (~15 bpm amplitude), no periodic changes. Uterine contractions 2-3 min apart, ~60-70 mmHg intensity. - Creasy & Resnik's Maternal-Fetal Medicine

Step 1 - Baseline Fetal Heart Rate

The baseline rate is the mean FHR over a 10-minute window, excluding accelerations and decelerations.

Finding	Rate	Significance
Normal	110-160 bpm	Reassuring
Bradycardia	<110 bpm	Non-reassuring / Abnormal
Tachycardia	>160 bpm	Non-reassuring (161-180) / Abnormal (>180)

The average FHR is around 140 bpm at term. Vagal tone maturation causes the rate to decrease progressively through gestation (155 bpm at 20 weeks → 140 bpm at term).

Step 2 - Variability

Variability refers to the normal beat-to-beat fluctuations in FHR. It reflects an intact neurological pathway from the fetal brain → vagus nerve → sinoatrial node. This is one of the most important features on the trace.

Type	Amplitude	Meaning
Moderate (normal)	6-25 bpm	Reassuring - healthy CNS
Minimal	1-5 bpm	Non-reassuring (may be sleep cycle, drugs, or early hypoxia)
Absent	<1 bpm	Abnormal - possible acidosis
Marked (saltatory)	>25 bpm	Abnormal if sustained >25 min
Sinusoidal	Smooth sine wave, 3-6 cycles/min	Abnormal - indicates severe fetal anemia

Causes of reduced variability: Fetal sleep (most common, up to 40 min), fetal acidosis/hypoxia, opiates/benzodiazepines, prematurity (<28 weeks), congenital heart block.

Step 3 - Accelerations

Accelerations = abrupt FHR rise >15 bpm for >15 seconds above baseline.

Reassuring sign - indicates healthy fetal CNS activity
Typically occur with fetal movements
In fetuses <32 weeks, the threshold is >10 bpm for >10 seconds
Absence of accelerations on an otherwise normal trace is of uncertain significance alone

Step 4 - Decelerations

Decelerations = drop in FHR >15 bpm for >15 seconds. They are classified by their timing relative to uterine contractions.

This diagram shows all three types compared:

Decelerations: Early, Variable, Late - Osmosis/Elsevier

Early Decelerations

Begin with the contraction and recover when the contraction ends (mirror image)
Caused by increased fetal intracranial pressure → vagal stimulation
Physiological - not concerning

Variable Decelerations

Abrupt onset and recovery (onset to nadir <30 seconds)
Variable in shape, duration, and depth
Not consistently related to contractions
Caused by umbilical cord compression
Presence of "shoulders" (small accelerations before and after) = fetus is compensating, not yet hypoxic
Loss of shoulders = fetus becoming hypoxic - more worrying

Late Decelerations

Begin at the peak of contraction, recover after the contraction ends
Smooth, uniform shape
Caused by uteroplacental insufficiency → fetal hypoxia
Always pathological

This shows reflex late decelerations on a real CTG (note FHR dips after each contraction peak):

Reflex Late Decelerations CTG - Creasy & Resnik's

And here is non-reflex late decelerations with absent variability - the most ominous pattern (fetal scalp pH was 7.07 in this case):

Non-reflex Late Decelerations with Absent Variability - Creasy & Resnik's

Prolonged Deceleration

FHR drop ≥15 bpm lasting ≥2 minutes but <10 minutes
2-3 minutes = non-reassuring
3 minutes = immediately abnormal (same as acute bradycardia)

Step 5 - Contractions

Assess using the tocogram (lower channel):

Normal labour: contractions every 2-3 min, lasting ~60 seconds
Max 5 contractions in 10 minutes
5 contractions in 10 min = tachysystole (oxytocin effect, reduces fetal recovery time)

Overall CTG Classification (NICHD / NICE)

Category I (Normal) - all features reassuring

Baseline 110-160 bpm
Moderate variability (6-25 bpm)
No late or variable decelerations
Accelerations present or absent
Early decelerations present or absent

Category II (Suspicious/Indeterminate) - one non-reassuring feature

Examples: tachycardia, minimal variability, recurrent variable decelerations with moderate variability, prolonged deceleration 2-10 min, late decelerations with moderate variability.

Category III (Pathological/Abnormal) - requires immediate action

Absent variability plus any of: recurrent late decelerations, recurrent variable decelerations, bradycardia
Or: Sinusoidal pattern

Quick Memory Aid - "DR C BRAVADO"

Letter	Feature
DR	Define Risk
C	Contractions
B	Baseline rate
R	Rhythm
A	Accelerations
V	Variability
A	Accelerations (confirm)
D	Decelerations
O	Overall impression

Management by Category

Category	Action
Normal (Cat I)	Continue monitoring
Suspicious (Cat II)	Correct reversible causes (position change, IV fluids, stop oxytocin, O2), fetal scalp stimulation, consider FBS
Pathological (Cat III)	Immediate delivery (aim within 30 minutes)

Sources: Creasy & Resnik's Maternal-Fetal Medicine, NICE NG229 Fetal Monitoring in Labour, Creasy & Resnik's Maternal-Fetal Medicine 8e pp. 732-750.

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