The preterm nonstress test: effects of gestational age and length of study

Duration of biophysical profile in periviable and very preterm low-risk pregnancies

BACKGROUND

In recent years, perinatal viability has shifted from 24 to 22 weeks of gestation at many institutions after improvements in survival in neonates delivered at the limit of viability. Monitoring these fetuses is essential because antenatal interventions with resuscitation efforts are available for patients at risk of delivery at the limit of viability. However, fetal monitoring using biophysical profiles has not been extensively studied in very preterm pregnancies, particularly in the periviable period (20 weeks 0 days to 23 weeks 6 days).

OBJECTIVE

This study aimed to (1) investigate whether the completion of biophysical profiles within 30 minutes is feasible in very preterm pregnancies, and (2) determine the average observation time required to achieve a score of 8 out of 8 in very preterm pregnancies from 20 weeks 0 days to 31 weeks 6 days.

STUDY DESIGN

This study prospectively evaluated biophysical scores in singleton pregnancies undergoing routine ultrasonography at or near viability from 20 weeks 0 days to 23 weeks 6 days (periviable or group I), 24 weeks 0 days to 27 weeks 6 days (group II), and 28 weeks 0 days to 31 weeks 6 days (group III). The results and duration of biophysical profiles were compared with those of a control group (32 weeks 0 days to 35 weeks 6 days) undergoing indicated fetal surveillance. Biophysical profiles were performed for all studied pregnancies until a score of 8 out of 8 was obtained. When >1 biophysical profile was obtained during pregnancy, each was analyzed individually. Pregnancies with fetal anomalies or obstetrical/medical indications for fetal well-being surveillance were excluded. Analysis of variance and post hoc Tukey tests were used for comparisons.

RESULTS

Data were collected for 123 participants, yielding 79, 75, and 72 studies for groups I, II, and III, respectively. The control group included 42 patients, yielding 140 studies. At 30 minutes, 80% (63/79) of the studies in the periviable group had a score of 8 out of 8, as opposed to 100% (140/140) in the control group (P<.001). The mean±standard deviation time in minutes to achieve a biophysical score of 8 out of 8 was 23.3±10.1 in the periviable group, as opposed to 9.4±6.5 in controls (P<.001). Extending the study to +2 standard deviations (43.6 minutes) in the periviable group resulted in 97% (77/79) of the scans scoring 8 out of 8 in the absence of adverse outcomes. In the other groups, a biophysical score of 8 out of 8 within 30 minutes was obtained in 97% (73/75) and 100% (72/72) in groups II and III, respectively; the mean±standard deviation times were 17.1±8.4 minutes (group II) and 13.1±7.3 minutes (group III). No adverse outcomes developed during the study participation in groups I to III.

CONCLUSION

Biophysical scores of 8 out of 8 can be successfully achieved in low-risk periviable pregnancies (20 weeks 0 days to 23 weeks 6 days) within an observation time longer than the standard 30-minute duration. The time required to reach a score of 8 out of 8 decreases as gestation progresses. We suggest adjusting the observation time for biophysical profile completion according to the gestational age.

Longitudinal evaluation of computerized cardiotocographic parameters throughout pregnancy in normal fetuses: a prospective cohort study

INTRODUCTION

The longitudinal cardiotocographic (CTG) changes throughout pregnancy in normal fetuses have never been fully described. We aimed at characterizing the evolution of CTG parameters in healthy fetuses, from 24 to 41 weeks of gestation.

MATERIAL AND METHODS

A prospective cohort study was conducted in singleton fetuses without structural abnormalities on second-trimester ultrasound. At least one CTG was performed in each of the following intervals: 24-26 weeks(+6d) , 27-29 weeks(+6d) , 30-32 weeks(+6d) , 33-35 weeks(+6d) , 36-38 weeks(+6d) and ≥39 weeks; tracings were analyzed by the OMNIVIEW-SISPORTO 3.6 system. Cases of preterm delivery, fetal death, birthweight under the 10th percentile, low five-minute Apgar, umbilical artery acidemia or neonatal intensive care unit admission were subsequently excluded.

RESULTS

A total of 1049 eligible tracings were obtained from 145 fetuses. There was a significant increase over time in average long-term variability (LTV), average short-term variability (STV), number of accelerations and uterine contractions. Conversely, fetal heart rate (FHR) baseline and number of decelerations decreased. A high inter-fetal variability was observed, but there was considerable intra-fetal consistency. Fetuses showing a marked decrease in FHR baseline and those with a marked increase in average LTV had a significantly lower birthweight. Cesarean section rate was significantly higher in cases with a decrease in average STV throughout gestation.

CONCLUSIONS

This prospective longitudinal study shows an evolution in computerized CTG parameters during pregnancy, indicating the need to adapt interpretation criteria based on gestational age. The high inter-fetal variability and considerable intra-fetal consistency suggests the possible value of using each fetus as its own reference in serial assessments.

Antenatal fetal assessment: contraction stress test, nonstress test, vibroacoustic stimulation, amniotic fluid volume, biophysical profile, and modified biophysical profile--an overview

Antenatal fetal assessment was introduced into the United States in the 1970s. The initial antepartum test, the oxytocin challenge test, later renamed as the contraction stress test, became the gold standard for fetal surveillance. Its labor intensive requirements and contraindications made it inapplicable to some high-risk pregnancies. Other testing schemes were developed subsequently, the nonstress test and its alternative, vibroacoustic stimulation, the semiquantitative assessment of amniotic fluid volume, the biophysical profile and its modified version, the modified biophysical profile. This article is a brief critical review of these testing methods and focuses on the following: (1) physiologic bases; (2) testing methodologies; (3) supportive evidence from randomized controlled and observational trials; and (4) areas needing further investigation.

Maternal position during non-stress test and fetal heart rate patterns

OBJECTIVE

The aim of the study was to determinate whether maternal position during the non-stress test (NST) in different weeks of pregnancy influences fetal heart rate patterns.

MATERIALS AND METHODS

A total of 1055 NST lasting 30 min were performed in 368 autochthonous mothers with low-risk pregnancies. On the basis of maternal position during the test we divided into three groups: reclining, sitting, and walking. The cardiotocographic parameters considered were: number of minutes of reactive NST with minimum length, number of fetal movements, fetal heart rate baseline, number of large accelerations, number of dubious NST, and number of variable decelerations.

RESULTS

Fetal heart rate patterns in low-risk pregnancies were studied using NST in different gestational ages and in different maternal positions. Differences in heart rate were found in relationship to both gestational age and maternal position. The minimum length of NST necessary to record at least three large accelerations was significantly different in relationship to both gestational age and maternal position. The number of fetal movements perceived by the mother was greater in the reclining position than in sitting or walking. Together with the progression of pregnancy, the number of dubious NST decreased in all subgroups, especially in the sitting position. The greatest number of variable decelerations was observed in the reclining position and it was increased with pregnancy progression. The NST duration did not vary greatly in the reclining position, but in the sitting position or during walking, the time taken to record the three large accelerations required to define the trace as reactive, decreased significantly with the progression of pregnancy

CONCLUSIONS

Non-stress test in sitting position or during walking should be encouraged because fetal reactivity is more quickly observed.

Multiple assessment techniques evaluate antepartum fetal risks

As technology has advanced, the field of antepartum fetal evaluation has grown. As reviewed here, a variety of options are available for use in the complicated pregnancy, including application of fetal heart rate monitoring techniques, noninvasive assessment of amniotic fluid volume, sonographic evaluation of fetal behavior, and Doppler assessment of fetal blood flow. It remains unclear which test is the best for any particular situation. The NST is the simplest test to perform but has a higher false-positive rate than the biophysical profile or the CST. These two tests also appear to demonstrate superior sensitivity, at the expense of increased testing time and cost. The application of vibroacoustic stimulation improves the specificity of the NST, while the addition of a sonographic assessment of amniotic fluid volume increases sensitivity and creates an acceptable alternative as a primary test. The limited biophysical profile, with the NST component initially omitted, provides some savings in time and cost without apparent loss of sensitivity or specificity when compared with the full profile. Ultimately, while one particular technique of fetal assessment may never prove to be the best, certain techniques may have advantages over others in particular clinical situations. Umbilical artery Doppler velocimetry appears to be a useful adjunct to other forms of testing, especially in the pregnancy at risk for intrauterine growth restriction and pre-eclampsia. Recent data have shown the biophysical profile predicts the onset of amnionitis in the setting of preterm premature rupture of membranes (PROM). The sonographic assessment of amniotic fluid volume is particularly important in the serial evaluation of the pregnancy complicated by fetal growth restriction. As new technology leads to innovative forms of testing, it is expected that these new tests of fetal status will similarly add to, rather than replace, the existing items in our armamentarium.

Computerised antenatal fetal heart rate recordings between 24 and 28 weeks of gestation

OBJECTIVE

To assess computerised fetal heart rate recordings between 24 and 28 weeks of gestation for gestation related differences.

DESIGN

Prospective, cross sectional observational study.

SETTING

Liverpool Women's Hospital.

POPULATION

112 women: 28 at 27 weeks, 30 at 26 weeks, 27 at 25 weeks and 27 at 24 weeks of gestation, respectively.

METHODS

Fetal heart recordings of 60 minutes duration were performed once in each pregnancy using the System 8000 fetal heart rate programme (System 8000, Oxford Sonicaid Ltd, Chichester, UK). For each gestational age, records were analysed for short term variation, basal heart rate, accelerations and time spent in high episodes.

RESULTS

The mean short term variation increased with gestation (P = 0.05). No record had a short term variation <4 msecs. There was no relationship between heart rate and increasing gestation. The mean number of accelerations per record increased with increasing gestation (P < 0.01). 20% of recordings showed no accelerations > 15 bpm. The mean duration spent in episodes of high variation increased with gestation (P = 0.05). 13% of recordings showed no time spent in high episodes. All fetuses had normal outcomes at delivery.

CONCLUSIONS

Absence of episodes of high variation or absence of accelerations is not an abnormal finding at lower gestations. The standard threshold of 4 msecs for short term variation appears to remain valid at lower gestations. These differences should be considered when using computerised CTG analysis at early gestations.

Nonstress testing and contraction stress testing

Antepartum fetal heart rate (FHR) testing, including the nonstress test and contraction stress test, has evolved in clinical usage over the past 3 decades. Although the nonstress test has become a standard of care in high-risk pregnancy, it has been modified by the use of fetal stimulation (vibroacoustic stimulation) and the addition of automated fetal movement recording (actocardiotocography). In all of its formats, antepartum FHR testing has been associated with reduction of preventable fetal loss. More recently, there have been attempts to improve test efficacy by computer-enhanced approaches.

The incidence of abnormal findings from intrapartum cardiotocogram monitoring in term and preterm labours

A retrospective analysis of 514 consecutive labours delivering 530 babies over a period of 18 months was conducted by a high-risk pregnancy team in a tertiary teaching unit to compare the incidence of abnormal findings from intrapartum monitoring between labours occurring before and at or after 34 weeks' gestation. Those delivered by elective Caesarean section, or Caesarean section at the onset of labour because of contraindications to labour and vaginal delivery, and those with congenitally malformed fetuses were excluded. Tracings were scored using the FIGO 1987 guidelines. Seventy-four labours and 83 babies delivered before 34 weeks, and 440 labours and 447 babies delivered after 34 weeks in the study. There was a slightly higher incidence of suspicious CTG tracings (33.7% versus 19.6%, OR 2.66, 95% CI 1.6-4.4) in the preterm group, due mainly to decreased baseline variability (p<0.001, OR 3.57, 95% CI 1.8-6.9), but the incidence of other pathological patterns did not differ. Using the same set of criteria for interpretation, there was a higher incidence of abnormalities from continuous cardiotocogram monitoring in the preterm group compared to term labours, but the intervention rate for fetal distress was not significantly increased. Appropriate interpretative criteria for intrapartum monitoring of preterm labours should be devised.

On intrapartum fetal monitoring

The currently advised conduct for intrapartum surveillance of the fetus is either intermittent auscultation of continuous electronic monitoring, depending on the physician's preference. This applies to all, normal or high-risk, conditions. The bases for this recommendation, a number of controlled studies comparing the two methods, showed no better neonatal outcomes and increased cesarean section rates with electronic fetal monitoring. A review of the works pertaining to fetal development of cardiovascular and central nervous systems and their response to various pathophysiologic conditions (in animals and humans) was carried out in an effort to find an explanation for this apparently uncongruous position. It was found that fetal responses to seemingly comparable conditions are radically different depending on age of gestation. Many authors have pointed this out for the human fetus. However, for interpretation of electronic fetal monitoring in labor, various standard, nondescriptive, confusing words are used to imply the need for rapid intervention. The complete lack of uniform interpretation has been shown in studies comparing interobserver and intraobserver variations. This may be the consequence of poor or superficial teaching of a tool that requires much study and hard work for useful application. The inescapable conclusion is unpleasant but inevitable: to use electronic fetal monitoring properly it is necessary to start a new learning of the physiology of the fetus, its changing evolution as pregnancy advances, its different responses under stress or distress, and the various ways these are represented in electronic fetal monitoring tracings. These efforts take dedication and time spent in labor suites collating tracings with neonatal condition. Only by doing this will it be possible to assist the laboring patients with a useful tool that, so far, has not been adequately applied because of insufficient understanding.

Nursing assessment and responsibilities in monitoring the preterm pregnancy

Evaluating women for pregnancy-related problems that may result in preterm birth frequently requires electronic fetal monitoring at early gestational ages. Caution is needed to interpret information correctly from the preterm fetal heart rate and uterine activity tracings. Interpreting fetal heart rate tracings from preterm fetuses requires knowledge of fetal physiologic development. Obtaining clear tracings of preterm uterine activity remains a challenge and heightens the importance of thorough nursing assessment, including inquiry about risk factors for pregnancy complications.

Quantitated fetal heart rhythm at 20, 32 and 38 weeks of gestation and dependence on rest-activity patterns

Quantitative parameters of fetal heart rate (FHR) were automatically analysed at 20, 32 and 38 weeks of pregnancy. FHR was obtained both by the fetal ECG method and by wide range Doppler ultrasound with autocorrelation. At 32 and 38 weeks, FHR was studied in relation to fetal rest-activity according to the fetal behavioural state concept (coincidence 1F and 2F). Basal fetal heart rate was significantly higher at 20 weeks of gestation than at 32 and 38 weeks. The number of accelerations increased significantly from 20 weeks to 32 and 38 weeks for C2F periods. Parameters of FHR variability, i.e. ID, ABB, LTI indices and bandwidth, were higher during periods C2F compared to periods C1F. Lowest values of all four parameters were found at 20 weeks gestation. The ID index, which is a measure of short-term variability increased significantly between 32 to 38 (C2F). The absolute values of ID, ABB and LTI were lower for ultrasound recordings than for the fetal ECG.

Fetal monitoring of twins

Perinatal mortality for twins is five to seven times that for singletons. This disparity is related to a variety of risk factors that may alter the health of the woman and her fetuses. With twin pregnancies, antepartum surveillance frequently begins earlier in gestation. Although extensive antepartum and intrapartum nursing care is required for women with twins, little has been published in the nursing literature to provide direction for caregivers. Factors such as zygosity and gestational age influence monitoring in twin pregnancies. Current clinical and technical nursing issues are reviewed and related to the antepartum and intrapartum electronic fetal monitoring of twins.