Despite compelling evidence demonstrating no neonatal benefit, the medicolegal climate in the United States requires obstetricians to integrate continuous intrapartum surveillance into their care of the pregnant laboring patient. Due to the setup of labor and delivery units and the team-oriented approach that exists in most facilities, nurses, residents, nurse midwives, and physicians may all be regularly involved in assessing the CTG. To communicate effectively in the event that an abnormal CTG exists and invoke an appropriate level of concern, standardized terminology is necessary.^3,4

In 1997, the National Institutes of Child Health and Human Development (NICHD) sponsored a Research Planning Workshop that addressed this issue. The workshop’s express purpose was to develop “a standardized and rigorously, unambiguously described set of definitions that can be quantitated” for electronic fetal heart monitoring, with the ultimate goal of producing a common language that would facilitate further investigational research examining the predictive value of electronic fetal monitoring and management strategies to recognize and reduce intrapartum fetal compromise.⁵ The American College of Obstetricians and Gynecologists (ACOG); the Association of Women’s Health, Obstetric and Neonatal Nurses; the Royal College of Obstetricians and Gynaecologists; and the Society of Obstetricians and Gynaecologists of Canada not only endorsed the definitions, but recommended new interpretations, definitions, and particular intrapartum management actions for some situations.

In April 2008, the NICHD, ACOG, and the Society for Maternal-Fetal Medicine (SMFM) jointly sponsored a workshop on CTG, or fetal heart rate (FHR) patterns. The goals of this workshop⁶ were (1) to review and update the definitions for CTG pattern categorization as compared with the prior workshop, (2) to assess existing classification systems for interpretation of particular CTG patterns and make new recommendations for a system to be employed in the United States, and (3) to make recommendations for research priorities as they relate to CTG. The intent of this article is to familiarize the reader with the resulting standardized, quantitative nomenclature that is recommended to describe intrapartum CTG to reduce miscommunication among providers caring for the laboring patient, as well as systematize the terminology used by researchers investigating intrapartum CTG. A new emphasis on interpretative systems and recommended management strategies, as set forth by the recent 2008 joint workshop, is also included and reviewed in detail.

Baseline fetal heart rate is the average fetal heart rate rounded to increments of 5 beats per minute during a 10-minute segment, excluding accelerations and decelerations and periods of marked variability (> 25 beats/min).

In any given 10-minute window, the minimum baseline duration must be at least 2 minutes (not necessarily contiguous), or else the baseline is considered indeterminate. In cases where the baseline is indeterminate, the previous 10-minute window should be reviewed and utilized to determine the baseline.

A normal FHR baseline rate ranges from 110 to 160 beats per minute. If the baseline FHR is less than 110 beats per minute, it is termed bradycardia. If the baseline FHR is more than 160 beats per minute, it is termed tachycardia.

Baseline FHR variability is determined in a 10-minute window and excludes accelerations and decelerations. A sinusoidal fetal heart rate pattern is defined as a visually apparent, smooth, sine-wave-like undulating pattern in FHR baseline with a cycle frequency of 3 to 5 per minute that persists for 20 minutes or more. A sinusoidal fetal heart rate pattern is incompatible with the definition of variability.

Variability is defined as fluctuations in the FHR baseline with irregular amplitude and inconstant frequency. These fluctuations are visually quantitated as the amplitude of the peak to trough in beats per minute, shown in Table 1.

Based on visual assessment, an acceleration is defined as an apparent abrupt increase in FHR above baseline, with the time from the onset of the acceleration to its acme less than 30 seconds. The increase is measured from the most recently determined portion of the baseline. The peak is 15 beats per minute or more above the baseline, and the acceleration lasts 15 seconds or more, but less than 2 minutes from the onset to the return to the previously determined baseline. In pregnancies of fewer than 32 weeks of gestation, accelerations are defined as having a peak 10 beats per minute or more above the baseline and duration of 10 seconds or longer.

Prolonged acceleration is 2 minutes or longer and less than 10 minutes in duration, with any acceleration lasting 10 minutes or longer constituting a change in baseline.

FHR decelerations are classified as late, early, or variable. The characteristics of each type of deceleration are described in the following paragraphs.

Based on visual assessment, late deceleration is defined as an apparent gradual decrease and return to the baseline FHR in association with a uterine contraction, with the time from onset of the deceleration to its nadir as 30 seconds or longer. The decrease is typically symmetrical in shape and is measured from the most recently determined portion of the baseline to the nadir of the deceleration. The deceleration’s timing is delayed, with the nadir of the deceleration occurring after the peak of the uterine contraction. In general, the onset, nadir, and recovery of a late deceleration occur after the beginning, acme, and end of the associated contraction, respectively.

Based on visual assessment, early deceleration is defined as an apparent gradual decrease and return to the baseline FHR in association with a uterine contraction, with the time from onset of the deceleration to its nadir as 30 seconds or longer. The decrease is typically symmetrical in shape and is measured from the most recently determined portion of the baseline to the nadir of the deceleration. Early decelerations are coincident in timing with uterine contractions, with the nadir of the deceleration occurring simultaneously with the peak of the uterine contraction. In general, the onset, nadir, and recovery of a late deceleration occur in a coincident fashion with the beginning, acme, and end of the associated contraction, respectively.

Based on visual assessment, variable deceleration is defined as an apparent abrupt decrease in FHR below the baseline, with the time from the onset of the deceleration to the nadir of the deceleration as fewer than 30 seconds. The decrease is measured from the most recently determined portion of the baseline to the nadir of the deceleration. Variable decelerations may or may not be associated with uterine contractions. The decrease from baseline is 15 beats per minute or greater and lasts 15 seconds or longer, but lasts less than 2 minutes from onset to return to baseline. When variable decelerations occur in conjunction with uterine contractions, their onset, depth, and duration may vary with each successive uterine contraction. Variable decelerations may occur in conjunction with other findings, the clinical significance of which requires further investigational research. Some examples include a slow return of the FHR after the end of the contraction, biphasic decelerations, tachycardia after variable deceleration(s), accelerations preceding and/or following (often referred to as “shoulders” or “overshoots”), and fluctuations in the trough of the deceleration.

Although some authors have suggested grading decelerations based on such factors as the depth or absolute nadir in beats per minutes and duration, the predictive value of these grading systems has not been sufficiently established and requires further investigation.

Standardization of terminology and subsequent categorization into 1 of 3 tiers should aid providers who are deciding whether the patterns are suggestive of a lack of fetal acidemia or alternately require intervention. Despite numerous studies demonstrating that inter- and intraobserver variability is high when CTG tracings are reviewed,^7,8 there is consensus that normal tracings classified as Category I indicate an absence of fetal acidemia.^6,9,10,11 On the other hand, acidemia may be present in up to 1 of 4 fetuses with abnormal or Category III CTG tracings.¹² Although expeditious action is indicated to either resolve the concerning aspects of the abnormal tracing or to move towards delivery, due to the low prevalence of intrapartum fetal asphyxia, even abnormal tracings have a well-recognized false-positive rate that in some instances can be greater than 90%.¹³

Patients with indeterminate tracings, classified as Category II, may ultimately be the most difficult to manage in clinical practice. A mainstay of the recommended strategy is close, continuous evaluation and assessment. These tracings may ultimately fit criteria for normal, Category I tracings as time passes or after subsequent evaluative strategies, at which point confidence in the nonacidemic status of the fetus may be gained. Alternately, indeterminate tracings may ultimately meet the criteria for abnormal, Category III tracings, in which case the imperative to resolve concerning aspects or move expeditiously towards delivery will become clear. Due to the potential uncertainty inherent in these nonpredictive tracings, a call has been issued for investigational research focusing on the relationship between such tracings and clinical outcomes.

This document attempts to familiarize the reader with recently proposed NICHD language in an effort to further advance the cause of utilizing common terminology and employing consistent, evidence-based, and simple interpretative systems among providers who use continuous CTG in their clinical practice. Personal review of the original NICHD workshop document cited below, along with any or all of the additional sources for this article, is strongly encouraged.