# Situation: A 27-year-old woman, gravida 2 para 1, is at 30 weeks' gestation and attends a prenatal visit at a hospital outpatient department. Her pregnancy has been uncomplicated, and her 4-year-old son comes with her. Yesterday she was unsure whether the baby had moved as much as usual, so a nonstress test (NST) is done today. Over 20 minutes, the tracing shows a baseline of 140 beats/min, moderate variability, no decelerations, and two accelerations, each rising 12 beats/min above baseline and lasting 12 seconds. How should the nurse interpret the result?

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> subject: Nursing Practice II — Maternal and Child Health Nursing

## 문제

Situation: A 27-year-old woman, gravida 2 para 1, is at 30 weeks' gestation and attends a prenatal visit at a hospital outpatient department. Her pregnancy has been uncomplicated, and her 4-year-old son comes with her.

Yesterday she was unsure whether the baby had moved as much as usual, so a nonstress test (NST) is done today. Over 20 minutes, the tracing shows a baseline of 140 beats/min, moderate variability, no decelerations, and two accelerations, each rising 12 beats/min above baseline and lasting 12 seconds. How should the nurse interpret the result?

## 보기

1. Nonreactive, because the tracing must run for 40 minutes
2. Nonreactive, because the rises are below 15 beats/min for 15 seconds
3. Reactive, because the criteria for her gestational age are met **✔ 정답**
4. Reactive, because the baseline is within 110 to 160 beats/min

**정답: 3**

## 해설

Before 32 weeks, an acceleration is a rise of at least 10 beats/min lasting at least 10 seconds; the 15 × 15 rule applies from 32 weeks. At 30 weeks, two accelerations of 12 beats/min for 12 seconds within 20 minutes make the test reactive, and the normal baseline and moderate variability are reassuring.

## 심화 해설

Interpretation of the NST at 30 Weeks

The correct interpretation is reactive, because the acceleration criteria for a fetus before 32 weeks of gestation are met. At 30 weeks, the tracing shows two accelerations of 12 beats/min lasting 12 seconds each within a 20-minute window. This satisfies the preterm standard of at least 10 beats/min for at least 10 seconds, so the result is reassuring rather than nonreactive.

The classic 15 beats/min for 15 seconds rule is a term-gestation criterion. Applying it rigidly to a 30-week fetus would mislabel a healthy tracing as nonreactive. Before 32 weeks, the fetal autonomic nervous system and myocardial responsiveness are less mature, so acceleration amplitude and duration are physiologically lower. The 10 × 10 threshold is the age-appropriate standard for this gestational period.

The baseline of 140 beats/min and moderate variability are also reassuring, but they are not the defining feature of reactivity. A normal baseline alone does not make a tracing reactive; the accelerations must meet the gestational-age-specific criteria. Key point! Reactivity is determined by accelerations, not by baseline alone. A fetus can have a normal baseline and still be nonreactive if accelerations are absent or inadequate.

| Gestational age | Acceleration amplitude | Acceleration duration | Reactive if |
| --- | --- | --- | --- |
| Before 32 weeks | ≥ 10 beats/min | ≥ 10 seconds | 2 accelerations in 20 minutes |
| 32 weeks and later | ≥ 15 beats/min | ≥ 15 seconds | 2 accelerations in 20 minutes |

The rationale for separate preterm criteria is supported by studies comparing acceleration patterns across gestational ages. Park et al. analyzed 2358 normal pregnancies and found that 10 bpm–10 s and 15 bpm–15 s accelerations differ systematically before and after 32 weeks, confirming that the stricter term rule is not interchangeable with the preterm rule [1]. Castillo et al. noted that using term criteria in fetuses between 24 and 32 weeks produces high rates of false nonreactivity in otherwise normal fetuses, emphasizing the need for age-appropriate interpretive standards [2]. Gagnon et al. further demonstrated that the 15 × 15 definition is applicable only after 30 weeks in fetuses with a basal heart rate of 128 beats/min or less, and that acceleration amplitude is inversely related to baseline heart rate from 30 weeks to term [4].

Watch out! A common error is to extend the monitoring time to 40 minutes before calling a tracing reactive. The standard initial observation window is 20 minutes; if no accelerations are seen, monitoring may be continued for an additional 20 minutes to account for fetal sleep cycles. However, in this case the accelerations are already present within the first 20 minutes, so there is no need to prolong the test. The tracing is reactive based on the two qualifying accelerations, regardless of the total duration of monitoring.

Glantz and Bertoia compared 10-beat and 15-beat criteria for NSTs performed at or before 32 weeks and found that the 10-beat threshold identifies reactive tracings without increasing adverse perinatal outcomes [3]. This supports the clinical safety of using the lower amplitude cutoff in preterm fetuses. The 12 beats/min rise in this scenario falls between the two thresholds, so it would be reactive by the 10-beat standard but nonreactive by the 15-beat standard. Because the fetus is at 30 weeks, the 10-beat standard is the correct one to apply.

The presence of moderate variability adds further reassurance because it reflects intact central nervous system oxygenation and autonomic regulation. Moderate variability is the single most important indicator of fetal well-being and absence of acidemia. When moderate variability coexists with qualifying accelerations, the probability of fetal compromise is very low, and no immediate intervention is required. The absence of decelerations also rules out cord compression or uteroplacental insufficiency patterns that would warrant further evaluation.References (research sources)

- [1]Difference of fetal heart rate accelerations based on 10 and 15 beats per minute.Research articlePark YS, Koh SK, Hoh JK, Park MI (2010) · DOI: 10.1111/j.1447-0756.2009.01150.x

- [2]The preterm nonstress test: effects of gestational age and length of study.Research articleCastillo RA, Devoe LD, Arthur M, Searle N, Metheny WP, Ruedrich DA (1989) · DOI: 10.1016/0002-9378(89)90113-0

- [3]Preterm nonstress testing: 10-beat compared with 15-beat criteria.Research articleGlantz JC, Bertoia N (2011) · DOI: 10.1097/AOG.0b013e31821d85e5

- [4]Patterns of human fetal heart rate accelerations from 26 weeks to term.Research articleGagnon R, Campbell K, Hunse C, Patrick J (1987) · DOI: 10.1016/s0002-9378(87)80042-x

## 임상 시나리오

NST Interpretation by Gestational AgePreterm vs. Term Acceleration Criteria
Before 32 weeks, a reactive NST requires 2 accelerations of at least 10 beats/min for 10 seconds within 20 minutes. The 15x15 rule applies only from 32 weeks onward.

At 30 weeks, two accelerations of 12 beats/min lasting 12 seconds each meet the preterm standard, making the tracing reactive. Baseline 140 beats/min and moderate variability are reassuring but do not define reactivity.

CautionDo not apply the term 15x15 criterion before 32 weeks; this mislabels healthy preterm tracings as nonreactive. Reactivity is determined by accelerations, not by baseline alone.

## 핵심 개념

- **Nonstress test (NST)** — A fetal assessment that observes heart rate accelerations with fetal movement to evaluate oxygenation and autonomic function.
- **Reactive NST** — Presence of two or more accelerations meeting gestational age criteria within a 20-minute window.
- **Acceleration** — A transient rise in fetal heart rate; before 32 weeks, at least 10 beats/min for 10 seconds; at 32 weeks and later, at least 15 beats/min for 15 seconds.
- **Moderate variability** — Fluctuation in baseline fetal heart rate of 6 to 25 beats/min, indicating intact autonomic regulation.
- **Baseline fetal heart rate** — Average fetal heart rate over 10 minutes, normally 110 to 160 beats/min.

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