Methodological considerations for each study have been discussed previously in the thesis under Methodological considerations. Results have been presented under Results for each study. A brief summary of results and interpretation will now follow for all studies included in this thesis.
8.1 SUMMARY OF RESULTS AND INTERPRETATION 8.1.1 Study I
There is a wide variation in first stage labor progression and more rapid labor progress should not be expected until at least 5-6 centimeter of cervical dilation regardless of parity. Among nulliparous Swedish women with a labor with spontaneous onset and normal maternal and neonatal outcomes, the median duration of the first stage of labor was faster than their counterparts in studies conducted on American and African cohorts. Important differences were noted among women with the slowest (95th percentile) labor duration compared to other study populations, women in the Swedish cohort had a larger distributions of labor durations.
The assumption of 1 cm per hour cervical dilation rate for the first stage of labor is not universally meaningful or applicable. There are differences in progression for women during first stage of labor in different populations. Labor progression varies widely and labors experiencing a prolonged first stage can still result in normal outcomes despite longer labors than what is considered to be prolonged according to previous knowledge. While one must acknowledge the limitation of describing
‘normal’ labor patterns in a sample with frequent interventions, the inclusion of women with interventions such as amniotomy, epidural, and oxytocin augmentation strengthens the generalizability of the findings to current obstetric practice.
8.1.2 Study II
We found that being obese was associated with increasing labor duration when interested in time difference at higher quantiles rather than median difference.
Especially in the age-stratified quantile regression analyses, the duration of active first stage labor for the younger obese group was more than one hour longer
experiencing a slow labor this may be the threshold for crossing the dystocia line, i.e.
indication for slow labor, which in clinical management results in augmentation with oxytocin or a caesarean delivery due to protracted labor. For some obese women, this may not always be a true indication, as the results in this study along with seminal findings suggest that they are more likely to have a longer duration of both active first stage of labor and total active phase of labor(163). Given the pronounced trend of increasing BMI among childbearing women and the effect modification of maternal age, it is of importance to clinically pay more attention to this group of women and regard the evidence that maternal anthropometrics may alter labor duration. In summary, assuming that population-based inferences are generalizable to subgroups within the population has the potential to mislead clinical decision making.
8.1.3 Study III
Among nulliparous women with spontaneous onset of labor, longer first stage of labor duration was associated with longer second stage of labor duration and with higher risk of operative delivery and cesarean delivery. From a clinical perspective, the findings of an association between increasing labor duration during the active first stage would be associated with longer second stage of labor seems logical.
Here we found how this association also was related to clinical context from other aspects. One hypothesis for the non-linear increase, i.e. a plateau identified after 12 hours is that women with longer first stage durations have higher likelihood for interventions in the second stage (i.e. active management, OVD, CD). This
phenomenon may reflect compliance to clinical guidelines and management during active labor and definitions of labor dystocia. Thus, this plateau is most likely the results of labor management norms and not a biological phenomenon.
Our study findings show that the relationship between the first and the second stage is important to consider when studying the associations between labor duration and subsequent birth outcomes. The results may inform future studies regarding how to consider second-stage duration, when investigating labor duration and subsequent birth outcomes, with applications in perinatal epidemiology.
8.1.4 Study IV
Modelling first stage duration either as a categorical or a continuous variable also enhanced understanding of its relationship to increasing duration through the whole course of active first stage and multivariable regression models allowed us to control for potential confounders. Informed by our findings, no clear threshold when a true risk appears for each individual is observed, rather the risk is increasing along with a cumulatively increasing duration from 5-10 hours for moderate neonatal outcomes, with a twofold observed increased risk for labors beyond 10 hours. The mediation analysis model addressed the questions: a) “Is second stage labour duration
mediating the association between active first stage of labour and adverse neonatal outcome?” or b) “Is the mediated effect of second stage of labour duration similar for moderate and severe neonatal outcomes?” Our results indicate that the risk of severe and moderate neonatal outcomes is slowly increases with longer active labour duration. Despite this evidence suggesting a cumulative effect, the absolute risk of poor neonatal outcomes remains is low. The results also indicate that first stage duration is independently associated with adverse neonatal outcomes since, given that only 21 % and 13 % was mediated through second stage duration for severe and moderate outcomes respectively. The cumulative evidence that many foetuses are resilient to even quite long active labour while few are sensitive to these same time conditions indicates the importance of future work identifying factors and/or phenotypes of foetuses possessing lower capacity to cope with longer active labour. Both direct and mediated pathways contributed to the remaining positive associations with a cumulative increasing labour duration and risk of adverse neonatal outcomes, with different patterns observed for moderate and severe neonatal outcomes. It indicates that a higher risk for adverse neonatal outcomes only partially can be explained by either a prolonged second stage or prolonged first stage of labour duration, hence the underlying biological plausibility for each
neonatal outcome needs to be considered in future analysis. Composite outcomes were created in order to achieve adequate power to study rare severe neonatal events without inclusion of non-life threatening, treatable conditions that are more likely to be related to mode of delivery (i.e. fractured humorous, shoulder dystocia).
Creating composite neonatal outcomes is not straightforward, underlying aetiology for the severe outcome intracranial haemorrhage (ICH) may be either inherited or acquired during labour, however the consequences of ICH is fatal and actions to avoid cases are at interest. Although we used a more clinically relevant composite neonatal outcomes our findings appear similar to previous publications. For this categorization of the composite neonatal outcomes to cause bias, if anything, would have made these risks an underestimation.