How to Calculate Montevideo Units
Montevideo Units are calculated as the sum of the amplitude (in mm Hg) above baseline of all uterine contractions occurring within a 10-minute window, measured by an intrauterine pressure catheter (IUPC). 1
Measurement Requirements
- An intrauterine pressure catheter (IUPC) or internal strain gauge must be used to obtain accurate pressure measurements—external tocodynamometry cannot provide the quantitative data needed for Montevideo Unit calculation 1
- The catheter measures intrauterine pressure in millimeters of mercury (mm Hg) 1
Calculation Method
Step 1: Identify all contractions occurring within a 10-minute period 1
Step 2: For each contraction, measure the peak amplitude (maximum pressure) above the baseline resting tone 1
Step 3: Add together all the individual contraction amplitudes from that 10-minute window 1
Example: If during 10 minutes there are 5 contractions with peak amplitudes of 50,45,55,50, and 50 mm Hg above baseline, the Montevideo Units = 50 + 45 + 55 + 50 + 50 = 250 Montevideo Units 1
Clinical Interpretation
- Normal adequate uterine activity is generally considered ≥200 Montevideo Units 1, 2
- During oxytocin induction, 91% of women who achieved vaginal delivery reached at least 200-224 Montevideo Units, and 40% reached ≥300 Montevideo Units 2
- For active-phase labor arrest, oxytocin augmentation should target a sustained contraction pattern of >200 Montevideo Units for at least 4 hours before considering cesarean delivery for failure to progress 3
Important Clinical Caveats
Montevideo Units have significant limitations in clinical practice 1:
- Uterine contractility assessed by Montevideo Units is of limited value in determining if a patient is in active phase of labor, as contractions inconsistently increase in intensity, frequency, and duration throughout the first stage 1
- No abrupt change in contraction characteristics occurs to distinguish when active phase has begun 1
- Cervical dilatation pattern remains the most reliable indicator for prospectively identifying the onset of active phase and assessing labor progression 1
- Contractile patterns have been studied extensively but have yielded little to facilitate differentiating normal from abnormal labor 1
Alternative Measurement: Alexandria Units
Alexandria Units represent an alternative quantification method that accounts for both contraction intensity and duration: Montevideo Units × mean duration of contractions 4. However, this is less commonly used in clinical practice than Montevideo Units 4.