Objective
Category II fetal heart rate (FHR) tracings are considered indeterminate; thus, improved risk stratification of category II FHR tracings is needed. We estimated whether the presence of meconium increased the risk of adverse neonatal outcomes.
Study Design
This study was conducted within a prospective cohort of 5000 women with singleton pregnancies who were admitted in labor at term. Pregnancies with category II FHR in the 60 minutes before delivery were included. FHR data were extracted by trained nurses who were blinded to clinical outcome. The exposure was the presence of meconium. The primary outcome was a composite neonatal morbidity defined as ≥1 of the following: neonatal death, neurologic morbidity, respiratory morbidity, hypotension that required treatment, and sepsis. Secondary outcomes were nursery admission, cord pH, 5-minute Apgar score, and components of the composite. Logistic regression was used to adjust for confounders.
Results
Of the 3257 women with category II FHR tracings, 693 women (21.3%) had meconium, and 2564 women (78.7%) did not. Meconium was associated with higher risk of the composite morbidity (adjusted odds ratio, 2.49; 95% confidence interval, 1.78–3.48) and increased risks of the secondary outcomes. The associations remained significant when infants with meconium aspiration syndrome were excluded. Thick meconium was associated significantly with the composite morbidity.
Conclusion
The presence of meconium is associated with an increased risk of neonatal morbidity in women with category II FHR pattern. This clinical factor may assist clinicians in managing category II FHR patterns in labor.
Category II fetal heart rate (FHR) tracings are encountered commonly during labor. Problems with interpretation of the category II FHR tracing, in part, stem from the diversity of FHR patterns that are included in this group. Additionally, the association between category II FHR tracings and fetal status and neonatal outcomes is poorly defined. One study found that increasing duration of FHR tracing classified as category II before delivery was associated with increased incidence of low 5-minute Apgar score and neonatal intensive care unit (NICU) admission. But, the vast majority of fetuses with a category II FHR tracing will have normal outcomes after delivery.
It has been suggested that other clinical factors should be considered in the evaluation and management of category II FHR tracings to improve the predictive ability of electronic fetal monitoring (EFM) for acidosis and outcomes. Meconium-stained amniotic fluid is found in 12% of all deliveries. Multiple studies have reported that meconium is associated with adverse neonatal outcomes that include fetal acidemia, low Apgar scores, and a need for neonatal resuscitation. Furthermore, the risk of neonatal morbidity in the setting of both meconium and an abnormal FHR tracing may exceed the risk associated with either factor independently. We performed this study among women at term with a category II intrapartum FHR tracing to estimate whether the presence of meconium increased the risk of adverse neonatal outcomes.
Materials and Methods
This observational study was conducted among the first 5000 women who enrolled in a prospective cohort study of >8000 women. The purpose of the parent cohort, which included all consecutive term singleton pregnancies in labor, was to examine the relationship between intrapartum EFM and perinatal outcomes. The Washington University in St. Louis Human Research Protection Office approved the study.
Trained research staff collected detailed data from participants’ medical records. Gestational age was calculated based on the woman’s first ultrasound examination in the pregnancy and last menstrual period. A woman was considered to have diabetes mellitus for the purpose of this analysis if she had a diagnosis of pregestational or gestational diabetes mellitus based on the National Diabetes Data Group criteria ; hypertension in pregnancy included chronic hypertension, gestational hypertension, or preeclampsia. Labor management information was also recorded and included oxytocin administration, epidural use, and mode of delivery. Chorioamnionitis was defined based on a clinical diagnosis of maternal intrapartum temperature >38.0°C associated with maternal or fetal tachycardia, fundal tenderness, or purulent amniotic fluid. The infant was considered small-for-gestational age if the birthweight was <10th percentile based on the Alexander growth reference. Meconium information that was obtained from the medical record included its presence and classification as determined by the obstetric nurse and primary obstetric care provider. Thin or thick meconium was documented based on subjective evaluation of meconium color and consistency. Neonatal outcome data that were abstracted from the medical record of the infant included umbilical cord arterial gas analyses, Apgar score at 5 minutes, nursery admission, and medical complications diagnosed during the postnatal hospital admission.
The obstetric research nurses were trained formally to review EFM patterns systematically using the National Institute of Child Health and Human Development criteria and were blinded to all clinical data. Participants were included in this study if they had a category II FHR tracing during the hour before delivery. This was defined as a category II FHR tracing for ≥40 of the 60 minutes before delivery. Forty minutes was selected as the duration to represent a predominately category II FHR tracing, because each tracing was analyzed in 10-minute increments in this study. Women with any 10-minute FHR pattern in the hour before delivery that was defined as category III were excluded, as were women whose FHR tracing before delivery was category II for <40 of the final 60 minutes. Additional exclusion criteria included unknown meconium status, multiple gestation, major fetal anomaly, scheduled nonlabor cesarean delivery, and gestational age <37 weeks.
The primary outcome of the study was composite neonatal morbidity, defined as ≥1 of the following events: infant death before hospital discharge, neurologic morbidity, respiratory morbidity, hypotension that required therapy, and suspected or confirmed neonatal sepsis ( Table 1 ). Each component of the composite morbidity was evaluated separately as a secondary outcome. Other secondary outcomes included higher acuity nursery admission, umbilical artery cord pH <7.1, and Apgar score of <7 at 5 minutes. Our higher acuity nurseries consist of a NICU (level IV) and a special care nursery (level II).
| Component | Definition |
|---|---|
| Death | Infant death before hospital discharge |
| Neurologic morbidity | Hypoxic-ischemic encephalopathy, seizures, or use of hypothermic therapy |
| Hypoxic-ischemic encephalopathy | One or more of the following: umbilical artery arterial pH <7.0, base deficit ≥16, need for respiratory support at 10 minutes of life, 5-minute Apgar score <5, and moderate-severe neonatal encephalopathy |
| Respiratory morbidity | Respiratory distress, transient tachypnea of the newborn infant, or need for ventilatory support |
| Respiratory distress | Clinically diagnosed by nasal flaring, subcostal and intercostal retractions, and a need for supplemental oxygen to maintain oxygen saturations >95% |
| Transient tachypnea | Neonatal respiratory rate >60/min with or without supplementary oxygen to maintain oxygen saturation >95% |
| Hypotension that requires therapy | Low blood pressure that requires vasopressor therapy |
| Suspected or confirmed sepsis | Neonatal symptoms that include respiratory distress, temperature instability, apnea, lethargy with or without an abnormal complete blood cell count (6-12 hours after birth with leukopenia or leukocytosis with an immature total neutrophil ratio of >0.2), and/or positive blood culture |
Characteristics of women with a category II FHR tracing were compared based on the presence or absence of meconium. Categoric variables were compared with the use of the chi-square test. Maternal age was compared with the use of the Mann-Whitney U test because this variable was not normally distributed based on the Kolmogorov-Smirnov test. Rates of the primary and secondary outcomes were estimated within groups. Multivariable logistic regression models for the primary and secondary outcomes were developed to adjust for potential confounders. Covariates that were associated with the presence of meconium in univariable analysis ( P < .10) were included in the initial model and were removed sequentially with the use of a backward stepwise approach based on the likelihood ratio test or >10% change in the adjusted odds ratio (aOR). Covariates that were considered in the model included nulliparity, gestational age category (37 0 -38 6 weeks; 39 0 -40 6 weeks; ≥41 0 weeks), chorioamnionitis, maternal obesity (body mass index ≥30 kg/m 2 ), previous cesarean delivery, hypertension, diabetes mellitus, and oxytocin use. We performed sensitivity analyses that excluded pregnancies that were complicated by meconium aspiration syndrome (MAS) and chorioamnionitis. MAS was defined as respiratory distress and transthoracic echocardiographic findings of elevated main pulmonary artery pressures. We conducted a stratified analysis based on the classification of meconium as thin or thick. We explored specific features within category II FHR patterns to identify whether meconium in the setting of specific features conferred risk.
We estimated the ability of the presence of meconium to predict adverse neonatal outcomes among women with a category II FHR tracing by calculating the sensitivity, specificity, and positive and negative predicted values. Similarly, we assessed the test characteristics of a predictive model that included individual FHR characteristics in our model in addition to meconium.
Tests with a probability value of < .05 were considered statistically significant. All statistical analyses were performed using STATA software (version 10.0 [special edition]; StataCorp, College Station, TX).
Results
Among the 5000 women who were admitted at term with a singleton gestation, 3257 women had a category II FHR tracing in the hour before delivery. Of the women with the category II FHR tracing, 693 women (21.3%) had meconium-stained amniotic fluid; in 2564 women (78.7%), meconium was absent ( Figure ).
Women with meconium were more likely to be nulliparous and at a more advanced gestational age, but less likely to have been diagnosed with hypertension or diabetes mellitus. Chorioamnionitis and operative vaginal delivery were more likely in pregnancies with meconium, although the use of oxytocin was less common among women with meconium than those without meconium ( Table 2 ).
| Variable | Meconium (n = 693) | No meconium (n = 2564) | P value |
|---|---|---|---|
| Maternal age, y b | 25 (21-30) | 25 (21-30) | .61 |
| Nulliparity, n (%) | 318 (45.9) | 1026 (40.0) | .01 |
| Gestational age, wk (%) | < .01 | ||
| 37 0 -38 6 | 156 (22.5) | 1032 (40.3) | |
| 39 0 -40 6 | 458 (66.1) | 1383 (53.9) | |
| ≥41 0 | 79 (11.4) | 149 (5.8) | |
| Race | .15 | ||
| Black | 463 (67.0) | 1659 (64.9) | |
| White | 129 (18.7) | 563 (22.0) | |
| Other | 99 (14.3) | 335 (13.1) | |
| Obesity (body mass index, ≥30.0), n (%) | 401 (57.9) | 1384 (54.0) | .07 |
| Smoking, n (%) | 104 (15.0) | 344 (13.4) | .28 |
| History of cesarean delivery, n (%) | 60 (8.7) | 179 (7.0) | .13 |
| Hypertension, n (%) | 91 (13.1) | 483 (18.8) | < .01 |
| Diabetes mellitus, n (%) | 17 (2.5) | 106 (4.1) | .04 |
| Chorioamnionitis, n (%) | 34 (4.9) | 56 (2.2) | < .01 |
| Oxytocin use, n (%) | 410 (59.2) | 1805 (70.3) | < .01 |
| Epidural, n (%) | 627 (90.5) | 2341 (91.3) | .50 |
| SGA infant, n (%) | 93 (13.4) | 354 (13.8) | .79 |
| Mode of delivery, n (%) | .03 | ||
| Spontaneous vaginal delivery | 602 (86.9) | 2303 (89.8) | |
| Operative vaginal delivery | 65 (9.4) | 167 (6.5) | |
| Cesarean delivery | 26 (3.7) | 94 (3.7) |
a Within the cohort of women with category II fetal heart rate before delivery
Among women with a category II FHR tracing, meconium was associated with an increased risk of neonatal morbidity (12.1% vs 5.3%; aOR, 2.49; 95% confidence interval [CI], 1.78–3.48). Each component of the composite, except neonatal death, was also more common in pregnancies with meconium-stained amniotic fluid. However, because of the rarity of the individual outcomes, only respiratory morbidity and suspected sepsis reached statistical significance. Higher acuity nursery admission, fetal acidemia, and Apgar score <7 at 5 minutes were also more likely in the presence of meconium ( Table 3 ). When we excluded transient tachypnea of the newborn from our definition of respiratory morbidity, the association between meconium and morbidity remained similar in direction and magnitude (data not presented).
| Variable | Meconium (n = 693), n (%) | No meconium (n = 2564), n (%) | Adjusted odds ratio (95% confidence interval) | P value |
|---|---|---|---|---|
| Composite morbidity b | 84 (12.1) | 135 (5.3) | 2.49 (1.78–3.48) | < .01 |
| Death | 0 | 2 (0.1) | — | — |
| Neurologic morbidity | 5 (0.7) | 8 (0.3) | — | — |
| Respiratory morbidity b | 48 (6.9) | 60 (2.3) | 3.22 (2.13–4.85) | < .01 |
| Hypotension that required therapy | 1 (0.1) | 0 | — | — |
| Suspected sepsis b | 63 (9.1) | 114 (4.4) | 1.97 (1.34–2.90) | < .01 |
| Higher acuity nursery admission b | 89 (12.9) | 149 (5.8) | 2.41 (1.75–3.32) | < .01 |
| Neonatal intensive care unit c | 17 (2.5) | 21 (0.8) | 2.84 (1.48–5.44) | < .01 |
| Special care nursery a | 72 (10.4) | 128 (5.0) | 2.10 (1.48–2.98) | < .01 |
| Arterial cord pH <7.1 c | 16 (2.3) | 29 (1.1) | 1.90 (1.02–3.55) | .04 |
| Apgar <7 at 5 minutes c | 27 (3.9) | 32 (1.2) | 2.76 (1.62–4.86) | < .01 |
a Within the cohort of women with category II fetal heart rate before delivery
b Adjusted for nulliparity, gestational age category, and chorioamnionitis
When the 18 neonates with meconium-aspiration syndrome were excluded, meconium remained associated with the composite morbidity (9.8% vs 5.3%; aOR, 1.7; 95% CI, 1.25–2.58), respiratory morbidity (4.9% vs 2.3%; aOR, 2.16; 95% CI, 1.37–3.42), and higher acuity nursery admission (10.6% vs 5.8%; aOR, 1.79; 95% CI, 1.27–2.53).
Exclusion of pregnancies that were complicated by chorioamnionitis from the main analysis yielded similar results; the associations between meconium and composite morbidity (9.1% vs 3.7%; aOR, 2.83; 95% CI, 2.00–4.01), respiratory morbidity (6.1% vs 2.0%; aOR, 3.66; 95% CI, 2.36–5.68), suspected sepsis (6.1% vs 2.8%; aOR, 2.33; 95% CI, 1.55–3.51), and higher acuity nursery admission (9.9% vs 4.2%; aOR, 2.70; 95% CI, 1.94–3.76) persisted.
In stratified results by meconium thickness, 53 women without this data were excluded. Higher rates of the composite neonatal morbidity, higher acuity nursery admission, and a composite of either arterial cord pH <7.1 or Apgar score <7 at 5 minutes were found in pregnancies with thick meconium. These outcomes were not increased in pregnancies with thin meconium, compared with pregnancies without meconium present ( Table 4 ).
| Variable | Meconium | |||||
|---|---|---|---|---|---|---|
| Thick (n = 384), n (%) | Adjusted odds ratio (95% CI) | Thin (n = 256), n (%) | Adjusted odds ratio (95% CI) | None (n = 2564), n (%) | Adjusted odds ratio (95% CI) | |
| Composite morbidity b | 57 (14.8) | 3.65 (2.49–5.36) | 22 (8.6) | 1.33 (0.75–2.37) | 135 (5.3) | Reference |
| Higher acuity nursery admission b | 60 (15.6) | 3.46 (2.39–4.99) | 24 (9.4) | 1.39 (0.80–2.39) | 149 (5.8) | Reference |
| Arterial cord pH <7.1 or Apgar <7 at 5 minutes c | 26 (6.9) | 3.09 (1.90–5.03) | 11 (4.3) | 1.77 (0.90–3.48) | 55 (2.2) | Reference |
a Within the cohort of women with category II fetal heart rate before delivery
b Adjusted for gestational age category and chorioamnionitis
We further examined individual FHR characteristics that were associated with the composite adverse neonatal outcome in our cohort of women with meconium. Tachycardia was more prevalent and accelerations less common in women with the composite neonatal morbidity compared with women without any components of the composite outcome. However, after controlling for chorioamnionitis, associations were not statistically significant. Variability and decelerations were not significantly different in those women with and without the composite neonatal morbidity ( Table 5 ). In contrast, the presence of meconium was associated with an increased risk of the composite adverse outcome among women with a category II FHR tracing, even in the setting of specific generally favorable FHR characteristics ( Table 6 ).
| Characteristic | Composite neonatal outcome, n (%) | Odds ratio (95% confidence interval) | Adjusted odds ratio (95% CI) b | P value | |
|---|---|---|---|---|---|
| Present (n = 84) | Absent (n = 609) | ||||
| Accelerations present c | 13 (15.5) | 171 (28.1) | 0.47 (0.23–0.88) | 0.55 (0.29–1.05) | .07 |
| Tachycardia d | 23 (27.4) | 67 (11.1) | 3.02 (1.67–5.32) | 1.49 (0.76–2.92) | .25 |
| Variability e | |||||
| Always moderate | 25 (29.8) | 212 (34.8) | Reference | ||
| Ever minimal/absent | 52 (61.9) | 371 (60.9) | 1.19 (0.70–2.06) | 1.01 (0.58–1.77) | .96 |
| Ever marked | 7 (8.3) | 29 (4.8) | 2.05 (0.81–5.15) | — | |
| Ever moderate | 51 (60.7) | 368 (63.4) | 0.89 (0.55–1.47) | 0.97 (0.58–1.62) | .90 |
| Accelerations present c or ever moderate variability e | 55 (65.5) | 426 (70.0) | 0.81 (0.49–1.37) | 0.96 (0.56–1.65) | .90 |
| Decelerations f | |||||
| No repetitive decelerations | 48 (57.1) | 390 (64.0) | Reference | ||
| Repetitive late | 2 (2.4) | 20 (3.3) | 0.81 (0.09–3.52) | — | — |
| Repetitive variable | 34 (40.5) | 212 (34.8) | 1.30 (0.79–2.15) | 1.33 (0.79–2.23) | .28 |
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