During labor, a contraction monitor provides healthcare professionals with continuous information about both uterine activity and the baby’s well-being. This technology helps to track the progress of labor and to identify any concerns, ensuring a safer experience for both parent and baby. Understanding how to interpret the readings from this monitor can offer valuable insights into the ongoing process.
The Monitor’s Display
A typical external contraction monitor uses two sensors. One sensor, an ultrasound transducer, detects the baby’s heart rate, while the other, a tocodynamometer (toco), measures the frequency and duration of uterine contractions. These sensors transmit data to a screen, which displays two lines. The top line represents the fetal heart rate, while the bottom line shows uterine activity.
The monitor’s screen is gridded, with the horizontal axis measuring time and the vertical axis indicating intensity or heart rate. Each small block on the horizontal axis represents 10 seconds, with bolder lines marking one-minute intervals. This allows for a clear display of labor progression.
Interpreting Contraction Data
The bottom line on the monitor, representing uterine activity, shows wave-like patterns indicating contractions. To interpret these, focus on three characteristics: frequency, duration, and intensity. Frequency measures how often contractions occur, calculated from the beginning of one contraction to the beginning of the next. For example, if two peaks are three minutes apart on the horizontal axis, the frequency is three minutes.
Duration refers to how long each contraction lasts, measured from the start of the wave to its return to the baseline. This is measured by counting the 10-second blocks a contraction spans; a contraction lasts between 30 to 90 seconds. Intensity, indicated by the height of the wave on the vertical axis, reflects the strength of the contraction. While external monitors can show relative intensity, the actual strength is best assessed by palpation or internal monitoring.
Understanding Fetal Heart Rate
The top line on the monitor displays the fetal heart rate (FHR), measured in beats per minute (bpm). A normal FHR baseline ranges between 110 and 160 bpm. This baseline is the average heart rate observed over a 10-minute segment, excluding periods of significant change.
Variability refers to the natural fluctuations in the FHR line, which indicates a healthy nervous system in the baby. Moderate variability, showing fluctuations between 6 and 25 bpm, is considered normal. Accelerations are temporary increases in the FHR, at least 15 bpm above the baseline for at least 15 seconds, often occurring with fetal movement and indicating a well-oxygenated baby. Decelerations, or temporary decreases in FHR, are also observed and can be classified as early, variable, or late, each carrying different implications.
Recognizing Important Patterns
Synthesizing information from both the contraction and fetal heart rate lines is important for understanding labor progression and fetal well-being. Normal labor progression shows contractions becoming more frequent, longer, and stronger, accompanied by a normal FHR baseline, moderate variability, and the presence of accelerations. Early decelerations, which mirror contractions and are benign, result from head compression.
However, certain patterns signal concerns. Late decelerations, characterized by a gradual decrease in FHR that starts after the peak of a contraction and recovers after the contraction ends, may indicate reduced blood flow to the placenta and fetal oxygen deprivation. Variable decelerations, which are abrupt drops in FHR of varying shapes and durations, are associated with umbilical cord compression. Recurrent or severe variable decelerations, or any late decelerations, warrant immediate attention from medical staff.