Abstract
Objective. The purpose of this study was to determine whether there is a relationship between the frequency of meconium-stained amniotic fluid (MSAF) and the duration of labor in term singleton gestation.
Methods. The clinical characteristics of women who delivered term singleton live newborns between 2001 and 2006 were examined. The cases involving neonates with major congenital anomalies were excluded.
Results. (1) The frequency of MSAF in term pregnancies was 18.4% (806/4376); (2) MSAF was found in only 2.8% (28/1008) of women who delivered by elective cesarean, but in 23.1% (778/3368) of women who delivered after the onset of labor (p < 0.001); (3) The longer the duration of labor (first stage, second stage, or total), the higher the frequency of MSAF (p < 0.001 for each); this remained significant after adjusting for other confounding variables such as parity, duration of rupture of membranes, gestational age at delivery, and mode of delivery (p < 0.001 for each).
Conclusion. MSAF was found in only 2.8% (28/1008) of women who delivered before the onset of labor, but in 23.1% (778/3368) of women who delivered after the onset of labor. The longer the duration of labor, the higher the risk of MSAF in term singleton gestation.
Introduction
Meconium-stained amniotic fluid (MSAF) is frequently observed in the labor and delivery unit and is a risk factor for neonatal meconium aspiration syndrome[Citation1–11], sepsis [Citation12], pulmonary disease and death [Citation1,Citation3,Citation5,Citation9,Citation13], subsequent development of cerebral palsy [Citation14], amniotic fluid infection [Citation15], chorioamnionitis [Citation5,Citation9,Citation12,Citation16–18], puerperal endometritis [Citation17], and dehiscence of perineal lacerations [Citation19].
Although the precise etiology of MSAF is still unclear, risk factors for MSAF include advanced gestational age at delivery, mode of delivery, increased duration of rupture of membranes (ROM), prolonged second stage of labor, and intra-amniotic infection [Citation8,Citation9,Citation15,Citation17,Citation20,Citation21]. Recently, Cheng et al. [Citation20] demonstrated that the longer the duration of the second stage of labor, the higher the frequency of MSAF (21% when the second stage was less than 1 h, and 28% when it was longer than 3 h). However, there is a paucity of information about the risk of MSAF when the neonate is born before the onset of labor (elective cesarean delivery) or according to the total duration of labor or duration of the first stage of labor. This study was conducted to examine the frequency of MSAF as a function of the duration of labor.
Methods
The study population consisted of consecutive pregnant women who delivered term (between 37 and 42 weeks of gestational age) singleton live newborns at Seoul National University Hospital between 2001 and 2006. Patients who delivered neonates with a major congenital anomaly were excluded. The presence or absence of meconium was determined at the time of delivery by visual observation. Elective amniotomy to evaluate the presence or absence of meconium is not routine practice in our institution.
Electronic medical records were reviewed for demographics, antenatal and intrapartum characteristics and pregnancy outcomes: maternal age, parity, presence or absence of labor and MSAF at delivery, premature rupture of membranes (PROM), duration of ROM, duration of labor (the first and second stages of labor and total), gestational age at delivery, birth weight, mode of delivery, Apgar scores, umbilical artery pH at birth and admission to the neonatal intensive care unit (NICU). These variables were compared according to the presence or absence of MSAF at the time of delivery. In this study, the women who delivered by elective cesarean section were not in labor. We did an extensive review of medical records and did not include women with regular uterine contractions with or without cervical change. The onset of labor was defined as regular painful uterine contractions based on history taken from the patients. The duration of labor was analyzed only in patients admitted before a cervical dilatation of 4 cm. The duration of the first stage was defined as the length of time elapsed during the progression of cervical dilatation from 4 cm to 10 cm. The duration of the second stage was defined as the length of time elapsed from full cervical dilatation to delivery of the fetus [Citation22]. This study, as well as the use of medical records for research purposes was approved by the Institutional Review Board of the Seoul National University Hospital.
Statistical analysis was performed using SPSS version 12.0 (SPSS Inc., Chicago, IL). Univariate analysis was conducted using Fisher's exact test or Student t-test as appropriate. Multiple logistic regression analysis was conducted for the adjustment of confounding variables. A probability value of <0.05 was considered significant.
Results
The frequency of MSAF in term pregnancies was 18.4% (806/4376). MSAF was found in only 2.8% (28/1008) of women who delivered by elective cesarean, but in 23.1% (778/3368) of women who delivered after the onset of labor (p < 0.001).
compares the clinical characteristics and pregnancy outcomes according to the presence or absence of MSAF at birth. Cases with MSAF had significantly higher maternal age, gestational age at birth and birth weight, higher rates of nulliparity, delivery after the onset of labor and umbilical arterial pH less than 7.0, and longer mean duration of ROM and duration of labor (total, first stage or second stage) than those without MSAF (p < 0.05 for each). In particular, 96.5% (778/806) of the cases with MSAF experienced labor before delivery. The rate of PROM was lower in patients with MSAF than in those without MSAF, but was not statistically significant (p > 0.1). Women with MSAF underwent cesarean delivery less frequently than those without MSAF (p < 0.001) because 74% (1008/1356) of women who delivered by cesarean had no labor before delivery, and the occurrence of MSAF was not frequent under such conditions. Neonates with MSAF had a significantly higher rate of admission to the NICU (p < 0.001).
Table I. Clinical characteristics and pregnancy outcomes of the study population according to the presence or absence of meconium staining of amniotic fluid at delivery.
shows the frequency of MSAF according to the duration of labor among women who delivered after the onset of labor. The longer the duration of labor (first stage, second stage, or total), the higher the frequency of MSAF (p < 0.001 for each, see ). This relationship persisted after adjusting for other confounding variables (p < 0.001, see ).
Table II. Relationship of various independent variables of meconium staining of amniotic fluid at delivery by multiple logistic regression analysis.
Figure 1. The frequency of meconium-stained amniotic fluid (MSAF) according to the duration of labor. The longer the duration of labor (first stage, second stage, or total), the higher frequency of MSAF (p < 0.001). A: The frequency of MSAF according to the total duration of labor. B: The frequency of MSAF according to the duration of the first stage of labor. C: The frequency of MSAF according to the duration of the second stage of labor.
Discussion
Principal findings of the study
(1) The frequency of MSAF in term pregnancies was 18.4%; (2) MSAF was found in only 2.8% of women who delivered by elective cesarean, but in 23.1% of women who delivered after the onset of labor; and (3) the longer the duration of labor (first stage, second stage, or total), the higher the frequency of MSAF.
MSAF according to the presence or absence of labor
In the current study, MSAF was found in only 2.8% of term pregnant women who delivered before the onset of labor. In contrast, MSAF was identified in approximately 25% of women who delivered after the onset of labor. This frequency is similar to those reported by other investigators [Citation8,Citation17]. Therefore, this finding is consistent with the view that the presence of labor is strongly associated with the passage of meconium into the amniotic fluid.
Why is labor a risk factor for MSAF?
Corticotropin-releasing hormone (CRH) derived from the placenta has been suggested to play an important role in the onset of labor [Citation23,Citation24]. Placental CRH may not only modulate myometrial contractility, but also stimulates the production of cortisol from the fetal adrenal gland [Citation23,Citation25]. It has been demonstrated that the cortisol concentration is significantly higher in term newborns delivered after the onset of labor than in those delivered before the onset of labor [Citation26]. Moreover, the umbilical cord blood cortisol concentration is significantly higher in newborns with MSAF than in those without MSAF [Citation25,Citation27]. Term fetal rats and fetal sheep can pass meconium before delivery via CRH, a known mediator of colonic motility [Citation24,Citation28]. Therefore, CRH itself and/or labor might stimulate meconium passage [Citation7,Citation23,Citation25,Citation28]. These observations suggest that MSAF might be a result of labor.
Another explanation is hypoxia. Parasympathetic stimulation of the fetal bowel due to hypoxic events or other stressors may induce premature bowel movements, leading to meconium passage in amniotic fluid [Citation10,Citation29–31]. Many previous studies have demonstrated that there is an association between MSAF and drop in the arterial cord pH value [Citation5,Citation10]. There is a significant inverse relationship in umbilical artery pH with the presence of labor in term newborns [Citation5,Citation32]. Our data also indicate that the umbilical cord pH <7.0 was significantly lower in newborns with MSAF than in those without MSAF (). However, the difference in the mean umbilical cord pH between the two groups was too small to reach significance. Furthermore, after adjusting for confounding variables, an umbilical arterial pH less than 7.0 was not an independent risk factor for MSAF ().
Gestational age and MSAF
The frequency of MSAF increased with advanced gestational age at delivery independent of the presence or absence of labor in the current study. This finding is consistent with the results of previous studies and gives support to the view that MSAF could be a physiologic event that represents normal gastrointestinal tract maturation under the fetal autonomic nervous system [Citation7,Citation9,Citation33–35].
MSAF and duration of labor and parity
Nulliparity itself may lead to an increased risk of obstetric complications [Citation36]. However, based on the inspection of demographic characteristics of the existing studies, it is not clear whether or not MSAF is associated with nulliparity [Citation10,Citation13,Citation18,Citation37]. The duration of labor of a nulliparous woman is significantly longer than that of a multiparous woman [Citation38,Citation39]. Several investigators have demonstrated that prolonged duration of the second stage of labor is associated with a higher risk of the occurrence of MSAF [Citation20,Citation40]. These observations suggest that the frequency of MSAF is higher among nulliparous women than among multiparous women after the onset of labor because of a longer duration of the second stage of labor in nulliparous women. The results of our study support the view and are consistent with findings of the previous studies [Citation6,Citation8]. Interestingly, these positive relationships between MSAF and the duration of the second stage of labor existed between MSAF and the duration of the first stage and total duration of labor ().
MSAF and PROM
PROM is a well-known risk factor for intrauterine infection [Citation41–55]. Several investigators have suggested that MSAF is highly associated with infection, including endometritis and chorioamnionitis [Citation16,Citation17]. Based on these reports, one would expect that the frequency of MSAF would be higher in the PROM group than the group without PROM. Unexpectedly, our study demonstrated that the frequency of PROM in cases with MSAF was similar to that in cases with clear amniotic fluid in the total population, but lower than those without MSAF in the labor group. In addition, two large-scale studies demonstrated that the frequency of PROM was significantly lower in the MSAF group than in the clear amniotic fluid group [Citation18,Citation41]. The mechanisms for this are unknown.
Strength of this study
We confirmed that the frequency of MSAF in women not in labor is drastically lower than in women with labor and clarified the association between MSAF and the duration of labor.
Unanswered questions and proposals for future research
Romero et al. [Citation15] demonstrated that the prevalence of a positive amniotic fluid culture was significantly higher in patients with MSAF than in those without MSAF among those who underwent transabdominal amniocentesis because of preterm labor and intact membranes. However, this kind of information is not available in pregnant women at term. We recently demonstrated that the presence and progression of labor are associated with an increased risk of a positive amniotic fluid culture for microorganisms, a more intense intra-amniotic response and histologic chorioamnionitis in term pregnancy with intact membranes [Citation56]. The relationship between the presence of MSAF and intra-amniotic infection or inflammation in term pregnancy should be examined in a large number of term pregnant women with the results of amniotic fluid analysis.
Acknowledgements
This research was supported, in part, by the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, DHHS.
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