The quality of intrapartum cardiotocography in preterm labour

Primary outcome measures

The quality of the FHR recordings during labour was determined by the median signal loss and by the percentage of cases exceeding the FIGO-criterion (>20% signal loss) for adequate quality, for both stages of labour [17]. The first stage of labour was defined as the time period after the beginning of the FHR recording and lasts until the beginning of the second stage. The starting point of the second stage of labour, defined as the moment of active maternal pushing, was looked up in the patient’s chart and compared with the patterns of increased uterine activity on the CTG when available [18]. If the time mentioned in the patient’s charts did not correspond with the changed patterns of uterine activity, the latter was used to determine the beginning of the second stage. The analysis of the FHR ended with the birth of the infant, or when the recording ended prior to birth, due to premature ending of the recording or operative intervention. In case a CS was performed, foetal monitoring continued until the mother was transferred to the operating room, which is standard procedure in Amsterdam UMC.

Secondary outcome measures

Detection of the maternal heart rate (MHR) instead of the FHR can occur during labour and was defined as the proportion of heart rate tracings with a sudden drop below 100 BPM or more than 20 BPM from the baseline for more than 30 s and a direct return to the baseline. MHR detection was determined through eyeballing meaning that MHR was measured roughly by sight, without the help of software. The total amount of MHR detection was counted and divided by the total duration of the recording. MHR detection is hard to distinguish from decelerations during the second stage and was therefore only examined during the first stage.

The time the foetus was unmonitored, was defined by the time between the ending of the recording and the time of birth and was calculated in case of operative interventions and premature ending of the FHR recording.

Determinants

We examined whether the quality of the FHR recordings was associated with GA, mode of delivery (MOD) and maternal overweight. GA was divided between 24–26 weeks and 26–28 weeks. MOD by vaginal delivery (VD) or secondary caesarean delivery (CS). Maternal overweight was considered a BMI>25 kg/m², calculated from pre-pregnancy weight.

Clinical characteristics

The following baseline characteristics were collected: maternal age (years), gravidity, parity, foetal birth weight (g), Apgar score after one and 5 min, pH of the foetal umbilical artery and duration of the second stage of labour.

Statistical analysis

Statistical analyses were carried out using SPSS Statistics 26.0 software. The Mann-Whitney U test, the Wilcoxon signed ranks test and the Chi-square test were used to compare baseline differences and significance. Data are presented as medians and interquartile ranges (IQR) and proportion of cases exceeding the FIGO-criterion. A p-value < 0.05 for a two-tailed test was considered statistically significant.

Baseline characteristics

Patient characteristics are presented in Table 1 and were equal for both groups. The numbers of recordings per mode of delivery are presented in Figure 3 .

Figure 3:

Number of recordings per mode of delivery.

Signal quality of the FHR recordings

A median signal loss of 13% for the first stage of labour was found. The median signal loss for the second stage was significantly higher, with 30% signal loss, see Table 2. Furthermore, approximately one third of the cases did not fulfil the FIGO-criterion for adequate quality during the first stage. This was 72% during the second stage of labour.

MHR registration did not contribute to foetal signal loss.

The median unmonitored time for VD was 1 min. In 10 cases, the unmonitored time was more than 10 min. This premature ending of the recording was mainly due to excessive signal loss, with a median signal loss of 23% for the first stage of labour. Consequently, the FHR was monitored with ultrasound instead of CTG. Data for the second stage of labour could not be retrieved for this reason.

Signal quality and gestational age

The quality of the FHR during the first stage of labour was almost identical between both GA groups (Table 2). Although not significant, signal loss during the second stage of labour was worse in the 24–26 weeks group in comparison to the 26–28 weeks group (38 vs. 28%, p-value = 0.34). For both GA groups, a statistical difference was seen in signal loss during the first versus the second stage of labour, at the expense of the second stage.

The proportion of cases not fulfilling the FIGO-criterion during the first stage of labour was lower in the 24–26 weeks group compared to the 26–28 weeks group (24 vs. 33%, p-value < 0.41). For the second stage, it was just the opposite, (80 vs. 69%, p-value = 0.50).

Signal quality and mode of delivery

CTG monitoring was performed until the mother was transferred to the operating room in case a CS was performed. No significant differences were seen in signal quality with respect to MOD, see Table 3. Of all CS, 40% were performed based on a non-reassuring FHR pattern, of which 41% did not fulfil the FIGO-criterion. In contrary, this was 20% in the other indications, which included infections (7.5%, n=4), alternative foetal presentation (13%, n=10) and abruptio placentae (18%, n=8). None were statistically significant.

Signal quality and maternal weight

For the first stage of labour, data on maternal weight was available for 29 mothers, see Table 3. Signal loss was more common in the overweight mothers (18 vs. 12%), however not significant. In the second stage of labour, data on maternal weight were available for only 12 mothers. Significance for the second stage of labour could not be analysed due to limited data on maternal BMI.

Principal findings

In this unique retrospective study, the quality of intrapartum cardiotocography in preterm labour before 28 weeks was studied. Our data demonstrate that FHR recordings of preterm foetuses are susceptible to signal loss during labour, particularly during the second stage of labour. This leads to an unmonitored foetus during a major part of labour.

Interpretation in light of other evidence

Although no other studies have examined intrapartum signal quality in preterm labour, studies have examined the quality in term foetuses. Signal loss for the external mode, in the first stage of labour varied between 5–13% vs. 10–19% during the second stage of labour. Direct mode proved to be of far better quality, with percentage of signal loss below 4% for both stages of labour [15, 19]. Our study demonstrated more signal loss, especially during the second stage of labour. Moreover, the direct mode cannot be applied in preterm foetuses since placement of the electrode at the delicate foetal skull should be avoided [12].

Two studies have examined signal quality in preterm foetuses during pregnancy. The first study examined preterm foetuses with a GA of 24–34 weeks and found that in 28% of the time the foetus was not continuously monitored [13]. The second study included pregnancies with a lower GA, 21–26 weeks, and found 55% signal loss with the external mode. Interestingly, when abdominal f-ECG was applied, signal loss decreased to 25% [14]. Unfortunately, it is still unknown if f-ECG can be used in preterm labour. Both studies correlate signal quality in pregnancy to gestation. The inadequate quality of FHR monitoring at younger gestation can be explained by the large volume of the uterine space in relation to the smaller foetus and foetal movements. However, our study cannot confirm this, most likely due to the small study population.

Signal quality with respect to MOD has not been studied before. Our study found that caesarean deliveries are frequently performed while the quality of the trace is inadequate. Notably, signal quality was even worse in the non-reassuring FHR pattern group compared to other indications. Almost half of the FHR recordings were classified as inadequate quality. This is distressing since the rising incidence in CS worldwide contributes to high maternal and foetal mortality and morbidity and should be avoided [20].

Other reasons for signal loss are due to the Doppler technique, which is susceptibility to noise. Main causes include improper placement of the transducer, maternal and foetal movement and detection of the MHR instead of the FHR. Some studies correlate signal quality to maternal BMI, as a result of increased distance between the transducer and foetal cardiac structures [14, 21]. Other reasons for inadequate signal quality include foetal cardiac arrhythmias and complicated decelerations [22].

Strengths and limitations

This is the first study investigating signal quality during preterm labour. Only few other studies explored signal quality of EFM in preterm foetuses. These studies addressed signal quality during pregnancy. Presumably, signal quality will be better during pregnancy since factors contributing to signal loss during labour, like maternal movement and contractions, are lacking. The opposite is true, compared to EFM in pregnancy, our results show superiority with remarkably higher quality signals [13, 14]. This can be explained by the support of a dedicated midwife with vast experience in monitoring. Another explanation might be the high-quality monitors and ultrasound transducers, as quality control is conducted every year. Two previous studies from our research group also demonstrated remarkably better-quality traces in term foetus and twins compared to similar studies, verifying this hypothesis [15, 16]. Nevertheless, signal loss is still impressively high and we can only assume signal quality in preterm foetuses during labour, with inferior quality transducers and a less experienced team, to be lower.

Unfortunately, our study was limited due to a small sample size. This explains why the correlation of signal loss with gestation and maternal overweight could not be confirmed, as expected from previous studies that do demonstrate a relationship [13, 14, 23].

Moreover, the median signal loss in our study was not evidently affected by recording of the MHR. Nonetheless, this does not mean that the MHR was not recorded since we were limited to eyeballing. Consecutive MHR recording for more than 30 s did not occur and therefore did not contribute to signal loss. MHR registration during second stage of labour was not measured. During this stage it is difficult to distinguish the MHR from foetal decelerations. However, monitoring the MHR instead of the FHR is likely to increase when the foetus descends. Previous studies found a median MHR registration of 0.70% for the first stage, up to 6% for the second stage of labour [23].

Practical and research recommendations

Worldwide, there is no consensus on labour management for extreme preterm birth considering mode of delivery and the application of foetal heart rate monitoring. Although a few low grade observational studies have been performed on this subject, strong evidence is lacking. The mode of delivery in spontaneous preterm birth did not affect neonatal mortality or morbidity, short and long term, in these studies. Since the incidence of perinatal asphyxia in extreme preterm infants is unclear, and the consequences of a CS for subsequent pregnancies are considerable, a vaginal delivery is still the preferred mode of delivery in the Netherlands and many other countries around the world [24, 25].

EFM is currently the only method to monitor the preterm foetus during labour and therefore is used worldwide as the standard method for monitoring. CTG opens for clinicians the option for additional information about foetal wellbeing and is therefore used in some countries as a tool to decide the optimal timing of intervention. However, clinicians must be aware that a non-reassuring FHR pattern is common during preterm labour and usually does not affect mortality rates [26]. Additionally, the quality of the FHR trace during labour is poor in a large proportion and signal quality cannot be improved with the direct mode. Other monitoring methods in the preterm foetus still needs to be explored, such as f-ECG [27].

In term foetuses the second stage of labour is most crucial phase, when the risk for perinatal asphyxia is the highest [28]. In preterm foetuses the incidence and contribution of perinatal asphyxia to the high mortality and morbidity rates remains unclear. Therefore, EFM in preterm labour is debatable as the standard method to monitor the foetal condition. Future research should focus on these subjects and is crucial to establish the value of FHR monitoring for the preterm foetus and the clinician.