From the Guidelines
The onset of labor is best identified by the acceleration of cervical dilatation, which can be determined through serial vaginal examinations at least every 2 hours, as this is the most reliable indicator of the active phase of labor. According to the most recent and highest quality study 1, the active phase of labor begins when the rate of cervical dilatation increases, regardless of the degree of dilatation achieved. This study, published in the American Journal of Obstetrics and Gynecology in 2023, 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, as contractions inconsistently increase in intensity, frequency, and duration over the course of the first stage.
Some key signs that may indicate the start of labor include:
- 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, or a sudden burst of energy before labor begins
It's essential to note that true labor contractions typically start irregularly but gradually become more regular, occurring every 5-10 minutes, lasting 30-60 seconds, and growing stronger over time. Unlike Braxton Hicks contractions, true labor contractions don't subside with rest or position changes. If you experience regular, painful contractions every 5 minutes for at least an hour, your water breaks, or you have heavy vaginal bleeding, contact your healthcare provider immediately as these indicate the start of labor or potential complications requiring medical attention 1.
The other studies provided 1 offer valuable information on labor and pregnancy but do not directly address the question of identifying the onset of labor as accurately as the study by Friedman et al. 1. Therefore, their findings, while relevant to the broader context of labor and pregnancy, do not supersede the recommendations based on the most recent and highest quality evidence regarding the signs of labor onset.
From the Research
Signs of Onset of Labor
The signs of onset of labor can be identified through various methods, including:
- Regular and effective contractions of the uterine myometrium 2
- Uterine contractions occurring at least 1 time in 10 minutes and persisting for more than 30 minutes before completion of 37 weeks of gestation without dilatation of the cervix, in the case of threatened preterm labor 3
- Three or more uterine contractions in 10 minutes associated with cervical modifications diagnosed by vaginal sonography (length ≤ 25 mm) 4
- Prelabor rupture of membranes, which may be followed by induction of labor within 24 hours 5
- Abnormal contraction patterns, such as shortening of the relaxation time, which can lead to severe asphyxia 6
Methods of Monitoring Uterine Activity
Uterine activity can be monitored using various methods, including:
- Manual palpation 2
- External tocodynamometry, which is the most widespread tool in clinical use due to its noninvasive nature and ability to time contractions against the fetal heart rate monitor 2
- Intrauterine pressure monitoring, which quantifies the strength of uterine contractions but is limited by its invasiveness and risk of complications 2
- Electrical uterine myometrial activity tracing, which is noninvasive and does not require ruptured membranes but is not yet widely used due to lack of access and high cost 2
- Internal tocography (IT), which provides objective information on quantization of uterine activity and has the ability to obtain a good quality trace in an obese, restless patient 6