There were 1
914 records of singleton births in Scotland for 1985-2004. We excluded 69
492 (6%) records where the gestational age was outside 37-43 weeks and 3677 (0.3%) with missing values for gestational age. Of the term deliveries, we excluded 899 (0.08%) perinatal deaths ascribed to congenital abnormality or rhesus isoimmunisation, 2054 (0.2%) stillbirths, and 44
000 (4.0%) non-cephalic deliveries. We also excluded 3184 (0.3%) records with missing values for time of birth, 334 (0.03%) records with unknown mode of delivery, 717 (0.1%) records where the deliveries were documented to have taken place in hospitals with fewer than 10 deliveries a year, and five records with inconsistent classification of perinatal death. Our study cohort consisted of 1
560 live births (some records had multiple exclusions), which corresponded to over 95% of all singleton term births in Scotland for 1985-2004. About half of the records excluded from the study cohort were preterm births of liveborn infants without congenital abnormalities (n=64
625, 52%). Among the study cohort, 72% of births occurred out of hours, whereas the figure was lower (70%) for preterm births over the period of study (P<0.001).
Women who delivered out of hours were younger, more likely to be primiparous, more likely to live in an area of low socioeconomic deprivation, and more likely to deliver in either low or high throughput units (table 1). They were more likely to have had labour induced and much less likely to be delivered by planned caesarean section. Their infants were also born at later gestational ages.
Table 1 Characteristic of cohort by timing of birth, Scotland 1985-2004. Figures are numbers (percentages) unless stated otherwise
There were 539 (0.05%) neonatal deaths in the study cohort (5.2 per 10
000 live births, 95% confidence interval 4.8 to 5.6). About half of these deaths were ascribed to intrapartum anoxia (n=273, 51%). The risk of neonatal death was 4.2 per 10
000 live births (3.5 to 5.0) during the working week and higher at all other times (table 2). The higher rate of neonatal death out of hours was explained by a significant excess risk of death ascribed to anoxia (unadjusted odds ratio 1.7, 1.3 to 2.3) and was similar in multivariable analysis. The magnitude (adjusted odds ratio of the increased risk of anoxic death was similar when we compared both 1701-0859 Monday to Friday (1.6, 1.2 to 2.2) and the weekends (1.7, 1.2 to 2.5) (table 3). When we stratified data by weekday or weekend, delivery between 1701 and 0859 was associated with an increased risk of anoxic death on weekdays (1.7, 1.2 to 2.3) but not at the weekend (0.7, 0.5 to 1.2) (interaction term P=0.005). When we excluded elective caesarean deliveries from the analysis, the association between timing of birth and the risk of neonatal death ascribed to anoxia was attenuated (table 3), but a significant association persisted. Similarly, the interaction between time of birth and day of the week was still present after we excluded elective caesarean births (interaction term P=0.01).
Table 2 Incidence of cause specific neonatal death by day and time of birth, Scotland 1985-2004
Table 3 Unadjusted and adjusted odds ratios (95% confidence interval) for cause specific neonatal death by day and time of birth
There was no evidence that the association between delivery out of hours and the risk of neonatal death ascribed to anoxia significantly varied over the study period (interaction term P=0.50); in relation to hospital throughput (interaction term P=0.23); for women who had a spontaneous labour compared with those in whom labour was induced (interaction term P=0.91); or for spontaneous vaginal deliveries compared with operative vaginal and emergency caesarean deliveries (interaction term P=0.73). After exclusion of births by elective caesarean section, the attributable fraction of neonatal deaths associated with delivery out of hours was 16.5% (1.3% to 29.3%) for deaths from all causes and 25.9% (4.8% to 42.3%) for neonatal deaths attributed to intrapartum anoxia. The nature of associations between delivery out of hours and the risk of neonatal death were unaffected when we performed clustered analyses accounting for births within hospitals (hospital level) or repeated deliveries in the same individual (maternal level).