Uterine Contractions

Measurement Parameters

   Two types of information can be ascertained from uterine contraction monitoring: quantitation of uterine activity (the strength of contractions), and contraction patterns (e.g. how many contractions, how often they are occuring). Assessment of contraction patterns is qualitative and can be performed with an external tocodynamometer or tocotransducer (Toco), whereas quantitative measurement of uterine strength requires an internal uterine pressure catheter (IUPC).

   Qualitative patterns include regular uterine contractions, polysystole, tachysystole, paired contractions, skewed contractions, tetanic contractions, and uterine hypertonus.
   
    In most normal spontaneous labors, contractions occur with a frequency of 2-5 minutes, and they may last between 30-60 seconds. The ascent and descent of the contraction are gradual and similar to one another. Contractions tend to become stronger and more frequent as labor progresses. Such a contraction pattern would be denoted as  regular uterine contractions, with a commentary on the frequency of the contractions (e.g., every 2-3 minutes).

   A normal contraction pattern is demonstrated in the picture above with contractions every 2-3 minutes.    

  Quantitative methods include Montevideo units (MVUs), Alexandria units, Active Planimeter units, Total planimeter units, and average rate of rise. Commonly, only MVUs are used in the US outside of research protocols.

   With an IUPC in place, quantitative data can be measured, most commonly using Montevideo units (MVU). A Montevideo Unit is the sum of the intensity of each contraction in a 10 minute period (in mmHG). Adequate uterine activity is defined as a contraction pattern that generates greater than 200 MVUs. Studies have shown that this threshold is adequate for 90% of labors to progress. Among women in spontaneous labors, more than 40% have MVUs > 300 mmHG.  Baseline pressure, or resting tone, is the uterine pressure in mm Hg while the uterus is relaxed.

Types of Uterine Contraction Patterns

   Tachysystole (or polysystole) is defined as 6 or more UCs in 10 minutes without evidence of fetal distress.

   Hypertonus is either an abnormally high uterine resting tone (>25 mmHG) or MVUs > 400.

   Hyperstimulation is either:

     1. persistent tachystole classically with evidence of fetal distress (late decelerations, lost variability)but now accepted as even without distress (the same as tachysystole), or

     2. a single UC lasting > 2 minutes may also be called a tetanic contraction, or

     3. UCs occurring within one minute of each other.  The most common cause of a tachysystolic, polysystolic or hypertonic contraction  pattern is  oxytocin or prostaglandins.

Uterine Hyperstimulation Video 1

Uterine Hyperstimulation Video 2

Uterine Hyperstimulation Video 3

Uterine Hyperstimulation Video 4

   Paired contractions are contractions that are coupled together, one after the other, then a prolonged gap in uterine activity occurs before the next set of paired contractions. This is sometimes thought to indicate cephalopelvic disproportion (CPD).

   A skewed contraction occurs when the crescendo and decrescendo of the contraction are not mirror images of one another. This relates to unequal relaxation of uterine muscle fibers and the clinical importance is not known.