The presence of meconium-stained amniotic fluid is associated with an increased incidence of adverse neonatal and long-term outcomes.1 It is usually only detected after rupture of the amniotic membranes and, therefore, most commonly in labor. Meconium that is passed by the fetus in labor is often interpreted as a sign of fetal “distress” or compromise.2 Unfortunately, unless clear amniotic fluid is seen in labor before the detection of meconium, its passage may have little to do with labor and be consequent upon a different process.
The absence of meconium or, at least, the presence of clear amniotic fluid is commonly considered reassuring. The original indication for early amniotomy with active management of labor was to detect meconium.3 In the presence of clear amniotic fluid, fetal heart rate auscultation is considered adequate fetal monitoring. Amniotomy in labor is also commonly performed to detect meconium where a fetal heart recording is unsatisfactory, despite the increase in fetal heart rate decelerations that may follow.4
Active management of labor involves early amniotomy on admission to the labor ward: at a mean cervical dilatation, at least in nulliparous women in The National Maternity Hospital, of 1.2 cm.5 Those women in whom clear amniotic fluid is then seen afford a unique opportunity to determine what has not been done before: to determine the associations and significance of the intrapartum passage of meconium.
The aims of this study were to 1) assess the frequency of the intrapartum passage of meconium and how often this is detected before delivery and 2) assess the relationship of intrapartum meconium with adverse neonatal outcomes and its use as a screening test for these, thereby investigating how reassuring clear amniotic fluid in labor should be considered.
MATERIALS AND METHODS
The cohort of women studied were those recruited to a randomized controlled trial in The National Maternity Hospital, Dublin, examining the effect of an admission cardiotocogram on neonatal morbidity. The results of this study have just been published.6 Women were only eligible for inclusion if they had an early amniotomy that showed clear amniotic fluid. Other exclusion criteria to the trial and, therefore, this cohort were refusal to participate, gestation of more than 42 weeks, known or suspected fetal growth restriction, poor obstetric history, and the woman or physician requesting routine continuous cardiotocography.
Women were randomized (sealed sequentially numbered envelopes containing treatment allocation) to 20-minute admission cardiotocography or intermittent auscultation only; continuous cardiotocography was then only undertaken if the admission cardiotocogram was considered abnormal, if meconium was subsequently detected, if fetal heart rate abnormalities were auscultated, or if the labor lasted more than 8 hours. Fetal scalp sampling was normally undertaken before cesarean delivery for fetal distress. The study was approved by the Ethics Committee of The National Maternity Hospital.
Between August 18, 1997 and May 1, 2001, 8580 women were recruited. Obstetric data were prospectively collected, although among infants admitted to the neonatal unit a consultant neonatologist (JM) collected neonatal outcomes from the hospital records after discharge. One hundred eighty-six (2.2%) were excluded from this analysis because of a breech presentation or a major congenital anomaly or because the gestation was less than 36 completed weeks or reached 42 weeks after randomization but before delivery. Therefore the cohort consisted of 8394 women.
Crude associations of potential ante- and postnatal risk factors were calculated using odds ratios (ORs) or means with 95% confidence intervals (CIs). Neonatal associations were then determined using also crude ORs and 95% CIs. These comprised an Apgar score of less than 7 at 5 minutes, neonatal seizures, moderate–severe metabolic acidosis (arterial pH less than 7.05 or venous pH less than 7.12 with a base deficit of greater than 12, which corresponds to more than two standard deviations below the mean), and admission to the neonatal unit. Meconium aspiration syndrome was defined as the need for ventilation of a neonate with typical radiographic features and in whom meconium was detected below the cords.
The use of the intrapartum passage or detection of meconium as screening tests for these adverse neonatal outcomes was then determined.
Of 8394 labors previously draining clear amniotic fluid, meconium was passed in 435 women (5.2%). It was seen before delivery in 211 (48.5%) and during delivery in 224 (51.5%) of these labors. The meconium was described as “thin” in 259 (59.5%) and “thick” in 176 (40.5%).
The associations of meconium in labor are shown in Table 1. Meconium is more likely to be passed with longer labors and oxytocin usage, in nulliparous women, and with epidural analgesia. It is also more common with increasing gestation. Meconium in labor is strongly associated with adverse neonatal outcomes. Eight infants (0.095%) developed meconium aspiration syndrome requiring ventilation, although in only five of these (63%) was meconium seen before delivery. Despite the strong associations between meconium and the parameters of adverse neonatal outcome, the sensitivity and positive predictive values of meconium passage (Table 2) for these are very poor, and worse for meconium that was identified before delivery (Table 2).
Meconium that was not identified until delivery did not have obviously different ante- or intrapartum associations. Particularly, it was not significantly more likely with instrumental delivery: 58 of 211 (27.5%) if meconium was seen before delivery and 76 of 224 (33.9%) if meconium was not seen until delivery (OR 0.74; 95% CI 0.49, 1.11). It was not also associated with significantly worse neonatal outcomes: for moderate–severe acidosis 20 of 191 (10.5%) and 33 of 208 (15.9%), respectively (OR 0.62; 95% CI 0.34, 1.12).
There was a marked difference in the frequency of adverse outcomes (except neonatal seizures) according to whether the meconium was described as thin or thick (Table 3), with infants with thick meconium faring worse. However, one patient (0.4%) with thin meconium nevertheless delivered an infant who developed meconium aspiration syndrome.
This cohort study has two principal findings. First, we demonstrate that in less than half of the infants who pass meconium in labor is it detected before delivery. The possibility that the passage of meconium that was not identified until delivery has an etiology different from that which was identified earlier is unlikely given that their associations, particularly instrumental delivery, are similar whether it was seen or not. The difference is likely to be in the detection, and this may simply be related to mechanical factors related to the leaking of amniotic fluid.
Most importantly, we show that most infants with a poor outcome do not pass meconium in labor: Therefore its sensitivity for adverse outcomes is extremely poor. This means that the absence of meconium intrapartum should not be considered reassuring. This has implications for practice on the labor ward, where artificial rupture of the membranes is frequently performed if there are cardiotocographic abnormalities and where methods for the intrapartum detection of meconium have been tried.7 In conjunction with recent data from sheep,8 it suggests that the passage of meconium may not be a direct response to hypoxia.
The most striking risk factors for meconium are a longer labor, epidural analgesia, and the use of oxytocin. We have not analyzed possible risk factors further using logistic regression because of the complexity of probable causal pathways leading to its passage and because meconium is a sign rather than a disease; we therefore wished to determine its significance in clinical practice.
Likewise we have not further investigated, with logistic regression, the relationship between meconium and cardiotocographic abnormalities. It has been demonstrated that if meconium is present and the cardiotocogram is normal, a good outcome can be anticipated.9 Although logistic regression would be possible, not all of our cohort received continuous cardiotocography, and those who did were often monitored only late in labor. This, therefore, could lead to misrepresentative conclusions about the relationship between meconium passed intrapartum and fetal heart tracings. The absence of complete cardiotocography data is unfortunate, but crucially, this does not affect our analysis of the use of meconium as a discriminator or screening test.
Meconium is passed rarely in this relatively low-risk cohort and aspirated in labor in 0.095% of the infants, although in three of these (37.5%) meconium had not been seen until delivery. This incidence is considerably lower than the overall incidence of meconium aspiration,10 although the actual incidence of meconium aspiration in which labor contributes to the causal pathway is likely to be higher because the fetus that passed meconium antepartum could subsequently aspirate it in labor.
Meconium is passed in labor relatively rarely. Although it is associated with an increased incidence of adverse neonatal outcomes, the majority of infants with such outcomes, excepting meconium aspiration, did not pass meconium in labor. In fewer still was the meconium detected, despite early artificial rupture of the membranes. The presence of clear amniotic fluid, even in late labor, is an unreliable sign of fetal well-being. This questions a key component of active management of labor: the use of early amniotomy to detect meconium to make decisions regarding electronic fetal monitoring. It also suggests that the presence of clear amniotic fluid should not influence management where the cardiotocogram is not reassuring.
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© 2003 The American College of Obstetricians and Gynecologists
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