From the Guidelines
The onset of labor is signaled by the acceleration of cervical dilatation, which can be determined through serial vaginal examinations, and is characterized by regular, painful contractions that increase in frequency and intensity, rupture of membranes, and the passing of the mucus plug. According to the most recent study published in the American Journal of Obstetrics and Gynecology in 2023 1, the active phase of labor begins when the rate of cervical dilatation increases, and this is the most reliable indicator of the onset of labor.
Key Signs of Labor Onset
- Regular, painful contractions that increase in frequency and intensity
- Rupture of membranes (water breaking)
- Passing of the mucus plug, which may appear as a blood-tinged discharge
- Lower back pain, pelvic pressure, diarrhea, and a sensation of the baby dropping lower in the pelvis (lightening)
The study emphasizes that assessing uterine contractions, either by palpation or using Montevideo Units, is of limited value in determining if a patient is in the active phase of labor 1. Instead, the dilatation pattern, as determined by serial vaginal examinations, is the most reliable indicator of the onset of the active phase and the normality of its progression.
Importance of Serial Vaginal Examinations
- Serial vaginal examinations should be performed at least every 2 hours to determine when the rate of dilatation increases
- The increased rate of dilatation signals the conversion to the active phase of labor
- This approach allows for the identification of abnormal labor patterns, such as protracted dilatation, arrest of dilatation, and prolonged deceleration phase
In the absence of clear diagnostic manifestations, the acceleration of cervical dilatation remains the most reliable indicator of the onset of labor 1. Therefore, healthcare providers should prioritize serial vaginal examinations to determine the onset of labor and monitor the progression of the active phase.
From the Research
Signs of Onset of Labor
The signs of onset of labor can be identified through various methods, including:
- Regular uterine contractions, which are a key indicator of labor 2
- Cervical dilation and effacement, which are signs of normal labor progress 3
- Changes in fetal heart rate patterns, such as accelerations and decelerations, which can be monitored using electronic fetal monitoring 4
- Uterine activity, including contraction frequency, duration, amplitude, and relaxation time, which can be monitored using various methods, including manual palpation, external tocodynamometry, and intrauterine pressure monitoring 2
Uterine Contractions
Uterine contractions are a crucial sign of labor, and their characteristics can be used to diagnose labor onset. These characteristics include:
- Frequency: regular contractions occurring at least once every 10 minutes 5
- Duration: contractions lasting for a certain period, typically 30 minutes or more 5
- Amplitude: the strength of the contractions, which can be measured using intrauterine pressure monitoring 2
- Relaxation time: the time between contractions, which can be used to assess uterine activity 2
Fetal Heart Rate Patterns
Fetal heart rate patterns can also be used to diagnose labor onset and assess fetal well-being. These patterns include:
- Normal baseline heart rate: between 110-160 beats per minute (bpm) 4
- Moderate variability: 6-25 bpm 4
- Accelerations: increases in fetal heart rate in response to uterine contractions 4
- Decelerations: decreases in fetal heart rate in response to uterine contractions, which can be classified as early, variable, late, or prolonged 4
Threatened Preterm Labor
Threatened preterm labor is a condition characterized by regular uterine contractions occurring before 37 weeks of gestation. The signs of threatened preterm labor include: