CTG Categories According to RCOG
The RCOG (Royal College of Obstetricians and Gynaecologists) uses a three-tier classification system for cardiotocography interpretation: Normal, Suspicious (also called Intermediate or Abnormal), and Pathological.
The Three RCOG Categories
The RCOG classification system categorizes CTG traces based on specific fetal heart rate (FHR) features including baseline rate, variability, accelerations, and decelerations 1, 2, 3.
Category 1: Normal
- Baseline FHR: 110-160 beats per minute
- Baseline variability: ≥5 bpm (moderate variability)
- Decelerations: None or early decelerations only
- Accelerations: Present (though absence alone doesn't make it abnormal)
- Management: Continue routine monitoring 2
Category 2: Suspicious (Abnormal/Intermediate)
This intermediate category includes traces with concerning features but not meeting pathological criteria:
- Baseline FHR: Bradycardia (100-109 bpm) or tachycardia (161-180 bpm)
- Baseline variability: Reduced (< 5 bpm for 40-90 minutes)
- Decelerations: Variable decelerations or single prolonged deceleration up to 3 minutes
- Accelerations: Absence of accelerations with otherwise normal trace
- Management: Increased surveillance, conservative measures (position change, hydration, discontinue oxytocin), consider expedited delivery if pattern persists or worsens 2, 3
Category 3: Pathological
These traces indicate significant fetal compromise requiring immediate intervention:
- Baseline FHR: Bradycardia < 100 bpm or tachycardia > 180 bpm
- Baseline variability: Absent variability (< 5 bpm for > 90 minutes), especially when combined with decelerations
- Decelerations: Recurrent late decelerations, recurrent variable decelerations with reduced variability, prolonged deceleration > 3 minutes, or sinusoidal pattern
- Management: Immediate intervention required; expedite delivery by operative vaginal delivery or cesarean section 2, 4
Clinical Application and Limitations
Important caveats: The RCOG system has moderate inter-observer agreement (κ=0.584) and moderate accuracy in predicting fetal acidemia, particularly during the second stage of labor 3. Studies show that 26.8% of acidemic fetuses may have unclassifiable CTG patterns versus only 7.9% of non-acidemic controls 3.
Comparative performance: When compared to other classification systems (FIGO, NICHD, Parer-Ikeda), the RCOG system demonstrates comparable but not superior predictive accuracy for neonatal acidemia, with an area under the ROC curve of approximately 0.72-0.73 4. The positive predictive value for pathological patterns ranges only 15-24%, though negative predictive values are reassuringly high at 95-99% 5.
Clinical pitfall: The RCOG classification was designed primarily for first-stage labor interpretation. During the second stage of labor, both RCOG and alternative classification systems show only moderate accuracy, with significantly more unclassifiable findings among acidemic fetuses 3. This limitation necessitates heightened clinical vigilance during active pushing.
Practical implementation: The system requires assessment of all four key FHR features simultaneously. A trace is classified by its most abnormal feature. For example, a trace with normal baseline and variability but recurrent late decelerations would be classified as pathological 1, 2.