From the Guidelines
The onset of labor is signaled by accelerating cervical dilatation, which can be identified through serial vaginal examinations at least every 2 hours. This is the most reliable method for prospectively identifying the onset of the active phase of labor, as contractile patterns and uterine contractions are inconsistent and of limited value in determining the start of labor 1.
To assess for the start of labor, it is essential to monitor cervical dilatation, as the active phase begins when the rate of dilatation increases from the latent phase to a more rapid progression. The following signs may also indicate the onset of labor:
- Regular contractions that increase in frequency, duration, and intensity
- Rupture of membranes ("water breaking")
- Passage of the mucus plug, a blood-tinged mucus discharge
- Lower back pain that radiates to the abdomen
- Pelvic pressure
- Diarrhea
It is crucial to differentiate true labor from false labor (Braxton Hicks contractions) by noting that true labor contractions continue despite movement or position changes, while false contractions often subside with activity changes or hydration. Women should contact their healthcare provider when contractions occur regularly, when membranes rupture, or if there is bright red bleeding 1.
The physiological changes that occur during labor are triggered by hormonal changes, including increased prostaglandin and oxytocin levels, which cause the cervix to efface (thin) and dilate (open) in preparation for delivery. The active phase of labor ends with the apparent slowing of dilatation, a deceleration phase, which is usually short in duration and frequently undetected 1.
In clinical practice, it is essential to prioritize the single most recent and highest quality study, which in this case is the 2023 study published in the American Journal of Obstetrics and Gynecology 1. This study provides the most up-to-date and reliable information on the signs of onset of labor, and its findings should be used to guide clinical decision-making.
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: