From the Guidelines
Labor onset is most accurately indicated by the transition from the latent to the active phase, characterized by an increase in the rate of cervical dilatation, which can be identified through serial vaginal examinations at least every 2 hours 1. The key parameters that indicate the onset of labor include regular uterine contractions, cervical changes, and other physical symptoms.
- Regular uterine contractions that progressively increase in frequency, duration, and intensity, typically occurring every 5-10 minutes, lasting 30-60 seconds, and becoming stronger over time.
- Cervical changes, including effacement (thinning) and dilation (opening), which can be assessed through vaginal examination, with the cervix beginning to dilate from closed to 10 centimeters throughout labor.
- The passage of the mucus plug, often called "bloody show," appearing as pink-tinged or blood-streaked mucus discharge.
- Rupture of membranes (water breaking), presenting as a gush or trickle of clear amniotic fluid from the vagina.
- Lower back pain, pelvic pressure, and gastrointestinal symptoms like nausea, vomiting, or diarrhea may also occur as labor begins. However, the most reliable indicator of labor onset is the increase in the rate of cervical dilatation, which can be identified through serial vaginal examinations at least every 2 hours 1. Assessing uterine contractions, for example, by palpation or using Montevideo Units, is of limited value in determining if a patient is in the active phase, as contractions inconsistently increase in intensity, frequency, and duration over the course of the first stage 1. In contrast, the dilatation pattern is alone as being reliable for prospectively identifying the onset of the active phase and the normality of its progression, provided it is being graphed serially in labor 1. It is essential to note that the active phase of labor begins at various degrees of dilatation when the rate of dilatation transitions from the relatively flat slope of the latent phase to a more rapid slope, and no diagnostic manifestations demarcate its onset, other than accelerating dilatation 1. Therefore, serial vaginal examinations at least every 2 hours are necessary to identify the onset of the active phase and monitor the progression of labor 1.
From the FDA Drug Label
The fetal heart rate, resting uterine tone, and the frequency, duration, and force of contractions should be monitored. The key parameters that indicate the onset of labor are:
- Fetal heart rate
- Resting uterine tone
- Frequency of contractions
- Duration of contractions
- Force of contractions 2
From the Research
Key Parameters Indicating Onset of Labor
The key parameters that indicate the onset of labor include:
- Regular uterine contractions 3, 4
- Cervical dilation and effacement 3, 5
- Parity and regional anesthesia status 3
- Uterine activity, which can be assessed through methods such as manual palpation, external tocodynamometry, intrauterine pressure monitoring, and electrical uterine myometrial activity tracing 4
Assessment of Uterine Contractions
Uterine contractions are a crucial indicator of labor onset, and their assessment is essential for diagnosing labor abnormalities 4. The frequency, duration, and intensity of contractions can be monitored using various methods, including:
- Manual palpation
- External tocodynamometry
- Intrauterine pressure monitoring
- Electrical uterine myometrial activity tracing
Cervical Length and Labor Onset
Cervical length, as measured by transvaginal ultrasound, can also be used to differentiate between true and false labor in term patients 6. A cervical length of ≤1.5 cm has been found to be a clinically optimal cutoff for predicting true labor, with high specificity and positive predictive value.
Labor Progression
Labor progression can be assessed by monitoring cervical dilation and effacement, as well as uterine contractions 3, 7. The expected labor progression after labor augmentation with oxytocin has been studied, and it has been found that labor progresses faster after effective uterine contractions are achieved 7. The time interval from 4 to 5 cm, 5 to 6 cm, and 6 to 10 cm dilation has been found to be shorter in women who receive oxytocin augmentation compared to those who do not 7.
Factors Affecting Labor Onset
Several factors can affect labor onset, including: