Diagnosing onset of labor: a systematic review of definitions in the research literature
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Description of included studies
We identified a total of 1683 potentially relevant citations (Fig. 1). Following title review, 549 were retained and review of these abstracts eliminated all but 117 studies. After full text screening, 62 studies were deemed eligible for inclusion in our review (see Table 1). Of the 62 included studies, 22 (35 %) were from the United States [25–44] and six (10 %) were from Germany [12–14, 45–48]. The remaining studies included four each from Italy [49–52] and Nigeria [53–56], three (5 %) each from Iran [57–59] and Norway [60–62], two each from Israel [63, 64] and South Africa [65, 66], and one each from Australia [67], Austria [68], Bahrain [69], Canada [70], France [45], India [71], Ireland [72], Jordan [73], Korea [74], Kuwait [75], New Zealand [76], Pakistan [77], Philippines [78], Saudi Arabia [79], South Korea [80], and Sweden [81]. Most of the included studies (n = 39, 63 %) were published between 2005–2013 (Fig. 2) [13, 14, 28, 30–34, 36, 38, 39, 42–48, 50–52, 55–59, 63, 67, 69–71, 73–77, 79–81]. The majority of studies were retrospective cohort studies (n = 29, 47 %) [2, 25, 27, 28, 30–36, 38, 40, 41, 43, 44, 49–51, 55, 61, 62, 65, 66, 70, 72, 80, 82, 83], while 29 % were prospective cohort studies (n = 18) [26, 29, 37, 39, 42, 46–48, 52–54, 56, 60, 67, 71, 74, 79, 81] and 11 % (n = 7) were randomized controlled trials or cohort [57–59, 69, 75, 77, 78]. The remaining eight studies (13 %) employed a range of qualitative, case control, mixed methods, or other research designs [12–14, 45, 63, 68, 73, 76]. Five studies (8 %) defined definitions of labor onset differently for nulliparous and multiparous women [36, 40, 41, 54, 66]. Of these five studies (8 %), four were published in 1986 or earlier [40, 41, 54, 66].
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How is the onset of labor onset defined?
Types of labor onset defined by the included studies
We classified the type of labor onset according to what the authors of the included papers said they were defining. In the 62 included studies, we observed four distinct types of labor onset including “active labor”, “latent or early phase labor”, “first stage labor” or simply “labor” without further specification, which we call “unspecified labor”. The majority of studies defined the onset of active phase labor only (n = 22, 35 %) [26, 27, 31, 35, 41, 42, 45, 50, 51, 53, 56, 57, 59, 61, 63, 65, 66, 71, 74, 77, 79, 83]. Three studies only defined the latent phase of labor (5 %) [40, 55, 76], while 11 studies only defined onset of the first stage of labor (18 %) (see Table 1) [32–34, 38, 47, 62, 68–70, 73, 78]. Approximately one quarter of studies provided only a definition for unspecified labor (n = 15, 24 %) [12–14, 25, 30, 36, 37, 39, 43, 44, 46, 48, 49, 52, 58]. With respect to studies that defined more than one ‘type’ of labor, 10 studies (16 %) provided a definition for both active and latent phase of labor [2, 28, 29, 54, 60, 67, 72, 75, 81, 82], while one (1.6 %) defined both active labor and unspecified labor [80].
Components of definitions of labor onset
Most studies (68 %) included measures of cervical dilation in their definition with only 20 studies omitting a specific measurement of dilatation from definitions of labor in their paper [12–14, 25, 29, 30, 32, 33, 35, 37, 40, 46–48, 61, 62, 70, 73, 76, 83] (Table 1). Regular painful contractions were also frequently referenced in definitions of onset of labor (71 %), with only 18 studies omitting mention of contractions [26, 27, 31, 36, 39, 41–45, 53, 55, 57, 59, 63, 68, 78, 79]. Studies also varied in their descriptions of the length and frequency of contractions at onset of labor. Twenty-one studies (34 %) included mention of either length or frequency of contractions in their definition of the onset of labor [29, 30, 32, 33, 35, 49–51, 54–56, 60–62, 66, 67, 69, 71, 74, 75, 83]. One study stated that onset of labor in general is also characterized by intact, rather than ruptured, membranes [28]. Below we outline how these commonly referenced components of labor definitions varied according to the type of labor defined.
Latent phase onset
Among the 14 studies that defined latent phase labor, 11 (79 %) included cervical dilatation in the definition. Onset of the latent phase of labor was defined using various measures of cervical dilation, most commonly <4 cm (n = 7, 50 %) [2, 28, 54, 60, 75, 81, 82]; however, ≤2 cm, [72] and <3 cm [29, 55, 67] were also included in definitions. One study provided different definitions for the end of latent phase labor according to parity, indicating that a cervical dilation of 3 cm marked the end of the latent phase of labor for primiparous women, while for multiparous women it was 4 cm [54]. Cervical effacement was included in the definition of latent phase of labor in three of thirteen studies (23 %). While two stated that effacement should be at least 80 % [29, 75], the third study defined latent phase labor onset as when the cervix has “minimal or no effacement” [55].
All thirteen studies (100 %) that provided definitions for the onset of latent phase labor included the presence of regular painful contractions in their definition [2, 28, 29, 40, 54, 55, 60, 67, 72, 75, 76, 81, 82]. Three studies (23 %) stated that during the onset of the latent phase of labor there should be at least one painful uterine contraction every 8–10 min [29, 54, 55], and one study stated that there should be at least two painful uterine contractions every 10 min [75]. The duration of each contraction was not included in these definitions. Only three studies (23 %) included other physiological symptoms in their definitions. These included bloody show [29, 72, 76] and fluid loss [72, 76], as well as gastrointestinal symptoms or irregular (non-repetitive) pain [72, 76].
Active labor
Of the studies that included a definition of the onset of active labor (n = 33), 27 (82 %) included cervical dilatation in their definition [2, 26–29, 31, 42, 45, 50, 51, 53, 54, 56, 57, 59, 60, 63, 65–67, 71, 74, 75, 77, 79–81]. Two (6 %) included ≥2 cm cervical dilation as the measure of labor onset [50, 51], ten (30 %) cited 3–4 cm [29, 45, 53, 54, 59, 65–67, 77, 79], while fourteen (45 %) included ≥4 cm cervical dilation in their definition of active labor onset [2, 26–28, 31, 42, 56, 57, 60, 63, 71, 74, 75, 80, 81]. Two studies (6 %) did not quantify the amount of dilation present at onset of active labor and stated rather that there should be contractions leading to “cervical change” [35, 83]. Four studies (12 %) characterized onset of active phase labor as the point at which the cervix begins to dilate >1 cm per hour [2, 41, 63, 79].
Cervical effacement was mentioned in six definitions (21 %) of onset of active labor [50, 51, 66, 72, 74, 81]. One study mentioned the cervix being generally effaced [81], one suggested that ≥75 % effacement was indicative of active labor [72], while three others considered the cutoff to be at least 80 % effaced [50, 51, 74], and finally one study referred to a “fully effaced” cervix [66].
Over half of the studies defining the onset of active labor included regular painful contractions in their definition (n = 20, 60 %) [2, 28, 29, 35, 50, 51, 54, 56, 60, 61, 65–67, 71, 72, 74, 77, 81–83]. Among the studies that defined onset of active phase labor, two indicated that contractions should be five minutes apart [66, 67], and two stated that there should be at least three contractions in ten minutes [71, 74], while two more suggested contractions should occur every 3–5 min [35, 83]. One study indicated that onset of active labor is characterized by contractions that are 20–25 s in length [71], while two studies (with the same first author) stated that contractions be >40 s long [50, 51]. Two studies included additional physiological symptoms in their definition of onset of active phase labor: fluid loss [72] and bloody show [29, 72].
First stage labor onset
Of the 11 studies that defined onset of the first stage of labor without referring to a particular phase [32–34, 38, 47, 62, 68–70, 73, 78], five (45 %) provided a specific cervical dilatation in their definition, including four that defined first stage labor onset when the cervix was 3–4 cm dilated [34, 38, 68, 69] and one study that used a cervical dilatation of ≥4 cm [78]. Three studies did not quantify dilation but stated that at first stage labor onset there should be “cervical change” [32, 33, 70]. Only one study that defined first stage labor included effacement in its definition (9 %), and mentioned only that there should be demonstrable effacement and dilatation of the cervix in their definition of first stage labor [38].
Most studies that defined onset of the first stage of labor included regular painful contractions in their definition (n = 9, 82 %) [32–34, 38, 47, 62, 69, 70, 73]. Only one study referred to duration or frequency of contractions at onset of first stage labor and indicated that contractions should be >40 s long [69].
Unspecified labor onset
Among the 16 studies that included a definition of labor that did not specify a phase or stage [12–14, 25, 30, 36, 37, 39, 43, 44, 46, 48, 49, 52, 58, 80], six (40 %) included a specific cervical dilatation in their definition. These six were evenly split between 2 cm [49, 52], 3–4 cm [43, 48], and >4 cm [36, 39]. Two studies included cervical effacement in their definition of onset of unspecified labor, stating that the cervix should be “partially” effaced [49] or ≥50 % effaced [52].
Twelve out of sixteen studies (75 %) that defined labor onset for an unspecified stage or phase of labor included regular painful contractions in their definition [12–14, 25, 30, 37, 46, 48, 49, 52, 58, 80]. Of these studies, four had the same first author [12–14, 46] and used a definition of onset of first stage labor that included multiple physiologic symptoms derived from a qualitative study on women’s experience of onset of labor at term [12]. Three studies diagnosed the onset of unspecified labor when one of the symptoms included contractions occurring at least three times in a ten-minute interval [30, 49, 52].
Definitions according to parity
Five studies provided a definition of labor onset that differed according to parirty [36, 40, 41, 54, 66]. One study indicated that latent phase labor and active phase began when the woman’s cervix was 3 cm or 4 cm dilatation for primiparous and multiparous women respectively [54]. Another suggested that labor (unspecified) began at 4 and 5 cm cervical dilatation for nulliparous and multiparous women respectively [36]. Two studies by the same authors reported that cervical dilatation was expected to occur at different rates based on parity (1.2 cm/h for nullips vs. 1.5 cm/h for multips) [40, 41].