Normal Range of Uterine Contractions in CTG
Normal uterine contractions are defined as no more than 5 contractions in a 10-minute period, averaged over a 30-minute window. 1
Defining Normal Contraction Frequency
The standard definition of normal uterine activity during labor is straightforward:
- ≤5 contractions per 10 minutes (averaged over 30 minutes) represents the upper limit of normal 1
- >5 contractions per 10 minutes (averaged over 30 minutes) is classified as tachysystole 1
This threshold applies to both spontaneous and stimulated labor, and the outdated term "hyperstimulation" should no longer be used. 1
Assessment Parameters Beyond Frequency
When evaluating uterine contractions on CTG, clinicians should assess multiple characteristics beyond just frequency: 1
- Rate and rhythm of contractions
- Duration of each contraction
- Intensity (though external transducers cannot always accurately measure this)
- Resting tone between contractions
An intrauterine pressure catheter (IUPC) should be used if accurate assessment of contraction strength is clinically necessary, as external transducers have limitations in measuring amplitude. 1
Emerging Evidence on Lower Thresholds
Recent research suggests that 4 contractions per 10 minutes may be a safer upper limit for identifying early signs of fetal compromise. 2 This study found that the region >4 but ≤5 contractions identifies the beginnings of worsening conditions in various measures of poor outcomes, including cord blood base excess, pH, and Apgar scores. 2 However, this has not yet been incorporated into formal guidelines, and the established threshold remains 5 contractions per 10 minutes. 1
Clinical Context and Tachysystole
Tachysystole must be qualified by the presence or absence of fetal heart rate decelerations, as this determines clinical significance and urgency of intervention. 1 Research confirms that ≥6 contractions per 10 minutes are significantly associated with fetal heart rate decelerations. 3
Important Caveats
- Contraction patterns do not reliably distinguish latent from active phase of labor or differentiate true from false labor 1
- Montevideo Units (≥200 units considered normal) have limited clinical value for determining labor phase or guiding management decisions 1
- Serial cervical examinations remain the gold standard for assessing labor progress, not contraction quantification alone 1