In 1958, Hon1 published the first set of data on 80 women who underwent successful electronic recording of “instantaneous fetal heart rate throughout labor and delivery.” Since then, electronic fetal monitoring (EFM) has come to remain the most common and widely used obstetric procedure in the United States2 with 84% of births being monitored annually.3 Despite its ubiquitous use, a meta-analysis of 12 randomized controlled trials noted that one additional cesarean delivery was performed for every 58 women monitored continuously and that 661 women needed to be monitored to avert one neonatal seizure.4 In addition, EFM use was found to increase the likelihood of vacuum, forceps, and cesarean deliveries.4
Despite the Cochrane systematic review4 not showing a benefit of EFM in reducing perinatal mortality, in a U.S. population-based study, we showed that EFM was associated with a substantial reduction in neonatal morbidity and mortality; this study also showed an increase in rates of operative vaginal and cesarean delivery.3 However, associations do not prove causation in retrospective EFM population-based studies, yet causation in such populations may be more likely if increasing use rates of EFM are also associated with declines in neonatal morbidity and mortality rates.
We examined the temporal trends between EFM and neonatal morbidity and mortality in the United States between 1990 and 2004. We also quantified the extent to which such temporal changes in EFM use were associated with changes in rates of primary cesarean delivery and neonatal morbidity and mortality.
PATIENTS AND METHODS
Our analysis examining temporal trends in the use of EFM in the United States was based on the vital statistics data.5 These data, ascertained through birth certificates, comprised all live births in the United States between 1990 and 2004 and were linked to infant deaths within the first year. These data files were assembled by the National Center for Health Statistics of the Centers for Disease Control and Prevention. Gestational age, reported in completed weeks, was predominantly based on a clinical estimate. Although the exact method of estimation of gestational duration based on a clinical estimate is unknown, this estimate is considered superior and more reliable than the menstrual estimate,6 also contained on the vital statistics data.
We restricted the study to women that delivered a singleton live birth between 24 and 44 weeks of gestation. The birth certificates contain details on maternal sociodemographic behavioral characteristics as well as data on medical and obstetric complications and outcomes of pregnancy. Obstetric complications, labor characteristics, and mode of delivery are recorded using a check-box format.
Of the 62,487,250 live births in the United States over the 15-year period (1990 through 2004), we sequentially excluded the following categories: multiple births (n=1,835,361) and neonates with congenital or chromosomal malformations (n=622,930). In addition, we excluded births with either missing gestational age (n=515,145) or those with gestational age at less than 24 weeks of gestation (n=176,310) or at 45 weeks of gestation or more (n=731,218). Finally, we excluded an additional 623,000 births with missing data on EFM use. Data for births in 2003 and 2004 include only those that were based on the 1989 version of the birth certificates. After all exclusions, 93% (n=57,983,286) singleton, nonmalformed live births remained for analysis.
We first examined the proportion of pregnancies that was monitored during labor and evaluated crude changes in EFM use between 1990 and 2004. This analysis was then initially stratified by gestational age at delivery as 24–27, 28–31, 32–33, 34–36, 37–41, and 42 weeks or greater. However, because the results were fairly similar across many of the groups, we subsequently combined the gestational age groups as 24–33, 34–36, and 37–44 weeks. To examine how changes in sociodemographic and behavioral factors (between 1990 and 2004) may have contributed to trends in EFM use, we carried out an analysis adjusted for confounders.
We examined how changes in EFM use may have affected changes in the risks of neonatal mortality and morbidity between 1990 and 2004. The risks of neonatal mortality (deaths within the first month) as well as early (up to 1 week) and late (7–27 days) neonatal deaths were examined. We examined the morbidity risks including 5-minute Apgar score lower than 4, cesarean delivery for fetal distress, operative vaginal delivery for nonreassuring fetal status, and neonatal seizures (at 34 weeks of gestation or after).
We estimated changes in EFM use between 1990 and 2004 as well as the associations between changes in EFM use on the risk of adverse neonatal mortality and morbidity from log-binomial regression models. From these models we estimated the risk ratio and 95% confidence interval following adjustments of confounders. The confounders that were adjusted in all models included maternal age (younger than 20, 20–24, 25–29, 30–34, 35–39, or 40 years or older), live-born parity (primiparous or multiparous), education (reported in completed years of schooling and grouped as less than 8, 8–11, or 12 years or more), maternal race or ethnicity (white, black, or other races and ethnicities), smoking, and marital status (single or married).
We did not seek ethics approval from the institutional review board for this study because the vital statistics data are completely deidentified and publicly available; therefore, the data do not qualify as human subjects research. All statistical analyses were carried out in SAS 9.3.
The rates of as well as the year-to-year change in the use of EFM during labor between 1990 and 2004 are shown in Figure 1. During the 15-year period, the rate of EFM use increased by 17%. In 1990, EFM was used in 73.4% of parturients, although it was used less commonly at 24–33 weeks of gestation (71.1%) than at 37 weeks of gestation or more (73.4%). By 2004, more than 85% of pregnancies were monitored in labor with EFM. A comparison of EFM use in 1990 compared with 2004 indicates that the rates increased sharply at all gestational ages (Table 1).
Changes in the distributions of maternal demographic characteristics, medical complications, and route of delivery between 1990 and 2004 are shown in Table 2. Figure 2 provides the gestational-age specific changes in EFM use in 1990, 1995, 2000, and 2004. Despite the temporal increase in the use of EFM overall, EFM use was higher at preterm gestational ages; at term gestations, the rate declined slightly.
Between 1990 and 2004, the temporal changes in overall neonatal mortality at 24–33, 34–36, and at 37 weeks of gestation or greater are described in Table 3. When adjusted for confounders as well as EFM use, the neonatal mortality rate decreased at all gestational age groups. The incremental benefit of EFM use on the decline in neonatal mortality was 5% at 24–33 weeks of gestation, 4% at 34–36 weeks of gestation, and 2% when at least 37 weeks of gestation. Similarly, improvements associated with EFM use were noted with early and late neonatal mortality.
Estimation of temporal changes with cesarean delivery and operative vaginal delivery for fetal distress is notable for a decrease, even when adjusted for confounders and use of EFM between 1990 and 2004 (Table 4). The incremental “benefit” of EFM indicates that because EFM use between 1990 and 2004 increased by 20% at 24–33 weeks of gestation, rates of both cesarean delivery and operative vaginal delivery have also increased during this period. In other words, had EFM use at 24–33 weeks of gestation not increased by 20% between 1990 and 2004, the overall decline in cesarean delivery for fetal distress over the same period would have been 13% (risk ratio 0.87). However, because of the temporal increase in EFM use at 24–33 weeks of gestation, the decline in cesarean delivery for fetal distress was only 9% (risk ratio 0.91). Apgar score lower than 4 at 5 minutes and neonatal seizure at 24–33 weeks of gestation both showed a temporal decline between 1990 compared with 2004. After adjustment for confounders and use of EFM, rates of low Apgar score declined for all gestational age groups between 1990 and 2004, although the reduction in neonatal seizures was seen only in the 24–33 weeks of gestation window.
Despite the prevailing uncertainty about its efficacy, the use of EFM for intrapartum assessment of fetal well-being is ubiquitous with 84% of more than 27 million births in the United States having been monitored.2,3 A meta-analysis of 12 randomized trials concluded that compared with intermittent auscultation, EFM was associated with an increase in operative vaginal deliveries and cesarean deliveries for nonreassuring fetal heart rate tracing; no change in the rates of cerebral palsy or neonatal mortality; and a lowering of neonatal seizures.4 However, we noted that EFM was associated with a decrease in early neonatal morbidity and mortality.3 The conflicting data between the meta-analysis and epidemiologic observations prompted us to evaluate temporal changes in the use of EFM and to estimate whether these trends were associated with changes in mortality and morbidity among U.S. births.
Four important findings emerged in our analysis. First, between 1990 and 2004, there was progressive incremental increase in the EFM use although it may have plateaued (Fig. 1). The potential reasons that 15% of births were not monitored include preference of intermittent auscultation,7,8 planned home births,9 precipitous labor10,11 and not using EFM at the limits of viability12 or admission to an hospital in advanced stages of labor. Despite the overall EFM use stabilizing in 2003–2004, it is noteworthy that among preterm pregnancies, monitoring was being increasingly used (Fig. 2). From 1990 to 2004, the greatest increase in EFM use was among the 24–33 weeks of gestation category, which is prudent because preterm births often have associated pathologic morbidity, may be more vulnerable to hypoxia and acidosis during labor if there is ischemic placental disease13,14 and have high perinatal mortality.15
Second, the temporal increase in EFM was associated with improvement in neonatal mortality. When adjusted for confounders, use of EFM was associated with a decrease in neonatal mortality by 2% (at 37 weeks of gestation or beyond) to 5% (for 24–33 weeks of gestation). The biologic plausibility for EFM being associated with a lowering of the neonatal mortality rate includes nonreassuring fetal heart rate tracing preceded by fetal hypoxia, acidemia, or death.3 In most cases, the use of EFM permits timely interventions with intrauterine resuscitation or, when needed, with operative delivery, thereby avoiding fetal acidemia.16–23
Third, use of EFM was associated with increased cesarean and operative vaginal delivery for fetal distress. After adjustment for confounders, EFM use was associated with an incremental increase in cesarean delivery for fetal distress by 2–4%. These findings corroborate the results of a meta-analysis.4 Potential reasons for the increase include the inter- and intraobserver variability in the interpretation of fetal heart rate tracings24,25 and lack of intrauterine resuscitation before proceeding with emergent delivery.26 Admittedly, there is some suggestion that medicolegal factors may also contribute to the increasing rate of cesarean delivery27; however, it remains uncertain whether the fear of litigation does increase the rate.28
Fourth, we noted that Apgar score less than 4 at 5 minutes was decreased but not neonatal seizures. We acknowledge that low Apgar score is a surrogate marker for adverse outcomes. Nevertheless, because low Apgar scores are associated with neonatal encephalopathy,29 and early neonatal death,30 we consider the link between increasing use of EFM and reduction in low 5-minute Apgar score to be important. This finding is also consistent with biological plausibility of monitoring in averting early neonatal mortality.3 The potential reasons for why the rate of neonatal seizure that were attributable to EFM was unaltered between 1990 and 2004 is that we were unable to segregate our cohorts into high compared with low risk3 and that not all neurologic injury to the newborn is the result of hypoxia.29 Aside from asphyxia, the causes of perinatal brain injury include subdural hemorrhage, intracerebral hemorrhage, cerebral infarct,31 hypoglycemia, hypocalcemia, malformation, infection or neonatal sepsis, and kernicterus.32 We speculate that increased use of EFM was associated with decreased rates of hypoxia-related seizures but not all seizures. There is also evidence that increasing EFM use is associated with a decrease in the incidence of hypoxic–ischemic encephalopathy term neonates.33
A few limitations of the study should be acknowledged. Studies based on birth and death certificates have been criticized for lack of quality of the data34; nonetheless, our findings are supported by biologic plausibility and corroborate the results of the meta-analysis of randomized controlled trials noting an increased rate of emergent deliveries with the use of EFM.4 Furthermore, misclassification of EFM use and the findings affected by residual confounding resulting from unmeasured factors (eg, antenatal corticosteroids, surfactant use) or those poorly recorded data cannot be overlooked.
This study adds further evidence to support the beneficial effects of the widespread use of EFM. The temporal increase in EFM use in the United States appears to be associated with a modest decline in neonatal morbidity and low 5-minute Apgar score.
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© 2013 by The American College of Obstetricians and Gynecologists. Published by Wolters Kluwer Health, Inc. All rights reserved.
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