In most (merged) context related patient groups, however, there is no proportional selleck chem distribution of patients (records) over the distinct parts of the day. A plausible explanation is that the professional interventions that affect the course of childbirth are spread unevenly over a 24 h day.12 This applies first, to referrals from the first line to the second line during labour, and second, to the augmentation of labour under the supervision of the second or third line. This explains why the number

of patients who reach the second stage of labour during daytime while being supervised by an obstetrician in the second/third line is proportionally greater than during the evening/night. Under these conditions, one cannot assume that the actual risk profiles of the ‘daytime group’ and the ‘evening/night group’ within the same (merged) context related patient group are equal to each other (figure 2). To complicate matters, the absolute numbers of adverse outcomes on which the differences in relative incidence are based usually are very small. Thus a simple calculation shows that, in the most recent time period, the ‘night/day difference’ in the relative incidence of perinatal mortality in the total group of (about 40) non-teaching hospitals (RR 1.17)

corresponds to three cases on an annual basis. Perhaps this is a good reason to consider the introduction of a new outcome variable that better matches the desired outcome of childbirth, for example: mother and child back home (in good health) within 1 week after birth. Shifts between (merged) context related patient (sub)groups not only occur within a certain time period, but also in successive periods. Often these shifts are the result of new professional insights, standards and habits that lead to other referral patterns and/or interventions. Examples include the changed

obstetric policy at breech presentations and at post-term pregnancies. With these types of changes over time the effect on the actual risk profile of the (merged) context related group can be assessed with reasonable accuracy. It is therefore Cilengitide easier to interpret a difference in the relative incidence of adverse outcomes by means of longitudinal comparisons than by means of transversal comparisons. Conclusion The complexity of the obstetric care system is not only the result of the multifactorial and dynamic character of the professional organisational contexts in which births take place. The size and the risk profile of the patient groups that are functionally related to these contexts are also in constant flux. This dynamic is to a large extent determined by professional intervention, at patient and also at policy level. All this makes it virtually impossible to demonstrate fixed patterns in the relationships between the separate contextual factors and the (adverse) outcomes of births.