Uterine contractions are the tightening and relaxing of the muscular wall of the uterus, known as the myometrium. These muscular efforts move the fetus toward the birth canal and are typically monitored to assess labor progression or the risk of premature delivery. Standard ultrasound imaging, which generates real-time pictures using sound waves, can capture this physical event. This capability, while not the primary method for measuring contractions, offers a unique visual perspective on the dynamics of the pregnant uterus.
Visualizing Contractions on the Screen
When a uterine contraction occurs, the change is readily visible on the ultrasound screen as a transient alteration in the appearance of the uterine wall. The myometrium becomes noticeably thicker and shorter in the area of the contraction. This focal thickening is the direct result of the muscle fibers shortening and tightening in a coordinated wave.
This visual phenomenon is distinct because it is temporary; the muscle wall returns to its pre-contraction thickness as the uterus relaxes. A Braxton Hicks contraction, for example, can be seen as a temporary, mound-shaped change in the placental contour. The transient nature of this focal myometrial thickening helps clinicians distinguish a contraction from other fixed masses, such as a uterine fibroid (leiomyoma). The observation is typically an incidental finding during a routine scan, rather than a planned measurement of labor activity.
Standard Methods for Monitoring Contractions
While ultrasound provides a visual confirmation of uterine activity, it is not the primary clinical tool for measuring the frequency, duration, or intensity of contractions. The standard method for external monitoring is external tocodynamometry (Toco), which uses a pressure sensor placed on the maternal abdomen. This device records the start, end, and frequency of contractions by detecting changes in the abdominal contour, but it cannot accurately measure the strength of the contraction.
Another common method is manual palpation, where a healthcare provider places a hand on the abdomen to feel the uterus harden during a contraction. This technique is subjective and requires the constant presence of a trained observer, but it offers a qualitative sense of intensity that Toco lacks. For the most precise measurement of contraction strength, an invasive intrauterine pressure catheter (IUPC) is used. The IUPC directly measures the pressure inside the uterus and is reserved for specific labor and delivery scenarios.
Clinical Scenarios Requiring Ultrasound Observation
Ultrasound observation of uterine contractions moves from an incidental finding to a deliberate diagnostic tool in several clinical situations. A primary scenario is the assessment of suspected preterm labor, where clinicians look for the effect of the contractions on the cervix. A contraction visible on ultrasound can cause the cervix to shorten, or efface, which is a concerning sign in a preterm pregnancy.
Observing the cervix during a contraction allows a provider to assess the dynamics of cervical change, which can be more predictive of labor progression than a static cervical length measurement alone. The degree of cervical shortening during a contraction can help differentiate between an inefficient contraction pattern and one that is likely to lead to delivery. Furthermore, this visual confirmation is used to evaluate the efficacy of medications administered to suppress preterm labor. If the contractions continue to be visibly strong or cause cervical change after treatment, it suggests the medication is not working effectively. The incidental finding of asymptomatic uterine contractions in women who already have a short cervix is another scenario where ultrasound provides valuable information.