Uterine contractions are the rhythmic tightening and relaxing of the uterine muscles during pregnancy and labor. These contractions help open and thin the cervix, allowing the baby to move through the birth canal. Monitoring these contractions provides information about labor progression and the well-being of the pregnant individual and baby.
Methods for Measuring Contractions
One way to assess contractions is manual palpation, where a healthcare provider or the individual can feel the abdomen to determine frequency, duration, and approximate strength. During a contraction, the uterus hardens and then softens, allowing for tactile assessment. This method can identify when contractions begin and end, but it provides a subjective measure of their intensity.
External tocodynamometry, often referred to as a “toco,” is a common non-invasive method using a pressure-sensitive device on the abdomen. Held in place by a belt, this device detects changes in uterine tension, recording contraction frequency and duration on a tracing. While convenient and widely used, the tocodynamometer does not accurately measure the intensity of contractions and can be affected by maternal movement or body mass.
For more precise contraction intensity measurements, an Internal Uterine Pressure Catheter (IUPC) may be used, though it is more invasive. This thin, flexible catheter inserts into the uterus, typically after amniotic sac rupture. The IUPC directly measures pressure within the uterine cavity in millimeters of mercury (mmHg), providing accurate data on frequency, duration, and intensity.
Understanding Contraction Readings
Contraction frequency is measured from the start of one contraction to the next. In early labor, contractions might be 5 to 20 minutes apart, gradually becoming closer as labor progresses. For instance, in active labor, contractions may occur every 3 to 5 minutes.
Duration refers to how long a contraction lasts, measured from beginning to end. Early labor contractions typically last 30 to 45 seconds; active labor contractions often extend to 45 to 60 seconds. As labor advances, contractions generally become longer, potentially reaching 60 to 90 seconds in the transition phase.
Intensity describes contraction strength, assessed subjectively as mild, moderate, or strong through palpation. With an IUPC, intensity is quantified in mmHg, with peak intrauterine pressure normally ranging from 40-80 mmHg during active labor. Adequate uterine activity, promoting normal cervical change, is generally considered greater than 200 Montevideo Units (MVUs) over a 10-minute period, calculated from IUPC readings.
When Contraction Measurement is Important
Contraction measurement helps differentiate true labor from false labor, often called Braxton Hicks contractions. True labor contractions follow a regular pattern, increasing in strength, frequency, and duration, persisting regardless of activity changes. Conversely, false labor contractions are irregular, tend not to increase in intensity, and may subside with rest or movement.
Healthcare providers use contraction measurements to monitor labor progression, tracking cervical dilation and effacement. A consistent increase in frequency, duration, and intensity usually indicates labor is advancing. This information guides decisions about care, such as when to administer pain relief or if interventions are necessary.
Abnormal contraction patterns can signal complications, requiring immediate medical evaluation. For example, uterine hyperstimulation (more than five contractions in 10 minutes or lasting over two minutes) can reduce placental blood flow, potentially leading to fetal distress or, rarely, uterine rupture. Similarly, strong, frequent contractions without cervical change could indicate a labor progression issue.
Individuals should generally contact a healthcare provider or go to the hospital when contractions become regular, lasting about one minute and occurring every five minutes for at least an hour. For those with previous births, a pattern of contractions every five minutes for an hour may signal a need for medical attention. Other reasons to seek immediate medical care include ruptured membranes (“water breaking”), heavy vaginal bleeding, or a noticeable decrease in fetal movement.