From the Guidelines
Uterine contractions do not reliably indicate the initiation of labor, as their intensity, frequency, and duration can be inconsistent and do not abruptly change to distinguish the active phase of labor. According to a recent study published in the American Journal of Obstetrics and Gynecology 1, 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. This is because contractions do not consistently increase in intensity, frequency, and duration over the course of the first stage of labor.
Some key points to consider when evaluating labor initiation include:
- The active phase of labor is characterized by a more rapid rate of cervical dilatation, which can be detected through serial vaginal examinations at least every 2 hours 1
- The onset of the active phase is marked by an increase in the rate of dilatation, rather than an abrupt change in uterine contractions 1
- Contractile patterns have been studied extensively, but have yielded little to facilitate differentiating real labor from false labor, or normal from abnormal labor 1
In clinical practice, it is essential to prioritize serial vaginal examinations and cervical dilatation patterns over uterine contractions when assessing labor initiation, as this approach is more reliable for identifying the onset of the active phase and detecting potential labor abnormalities 1.
From the FDA Drug Label
Antepartum Oxytocin Injection, USP (synthetic) is indicated for the initiation or improvement of uterine contractions, where this is desirable and considered suitable, in order to achieve early vaginal delivery for fetal or maternal reasons
- The presence of uterine contractions does not necessarily indicate labor initiation, as oxytocin can be used to induce or stimulate labor.
- Uterine contractions can be a result of oxytocin administration, which is used to initiate or improve contractions, but this does not directly answer whether uterine contractions indicate labor initiation.
- The drug label does not provide a direct answer to the question, as it discusses the use of oxytocin to induce or stimulate labor, but does not define what constitutes labor initiation 2, 2.
From the Research
Uterine Contractions and Labor Initiation
- Uterine contractions are a key indicator of labor initiation, as they are a necessary component of the labor process 3, 4, 5.
- Regular uterine contractions, in addition to dilation and effacement of the cervix, are used to identify normal labor 3.
- A study found that 12 contractions or more per hour at term is a meaningful signal that true labor has either begun or is imminent 4.
- Uterine contractions are produced through a complex interplay of hormonal, mechanical, and electrical factors, and are measured using various methods, including manual palpation, external tocodynamometry, and intrauterine pressure monitoring 5, 6.
Differentiating True from False Labor
- Cervical length measurement by transvaginal ultrasound can be used to differentiate true from false labor in term patients presenting for labor check 7.
- A cervical length of ≤1.5 cm was found to be the most clinically optimal cutoff for predicting true labor, with the highest specificity and positive predictive value 7.
- Uterine contractions, in combination with cervical length measurement, can be used to assess labor progress and diagnose labor abnormalities 3, 7, 5.
Assessment of Uterine Contractions
- Accurate assessment of uterine activity during labor is crucial for the management of protracted labor and the diagnosis of inadequate uterine activity 5.
- Various methods exist for evaluating uterine activity during labor, including manual palpation, external tocodynamometry, intrauterine pressure monitoring, and electrical uterine myometrial activity tracing 5.
- New monitoring methods, such as electrical uterine monitoring, are being developed to improve the assessment of uterine contractions and labor progress 5, 6.