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
A normal contraction pattern
is demonstrated in the picture above with contractions every
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.
Uterine Contraction Patterns
Tachysystole (or polysystole)
is defined as 6 or more UCs in 10 minutes without evidence of fetal
Hypertonus is either an
abnormally high uterine resting tone (>25 mmHG) or MVUs >
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
UCs occurring within one minute of each other. The most
cause of a tachysystolic, polysystolic or hypertonic
contraction pattern is oxytocin
Uterine Hyperstimulation Video 1
Uterine Hyperstimulation Video 2
Uterine Hyperstimulation Video 3
Uterine Hyperstimulation Video 4
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
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.