Objective: To assess the impact of the day of birth on twin mortality in a population sample.
Methods: We analyzed weekend versus weekday twin births from the United States national twin birth data for the periods 1989-2002. We computed adjusted hazard ratios (HR) and 95% confidence intervals (CI) to assess the association between infant mortality and weekday of birth using the Cox proportional hazards model.
Results: The crude rates for all types of mortality were found to be significantly higher for twins born on weekends than on weekdays. After adjustment, only post-neonatal mortality risk was higher on weekends as compared to weekdays [Hazards ratio (HR)=1.19, CI: 1.04, 1.36]. Twins of white mothers were at greater risk for neonatal death (HR=1.16, CI: 1.08, 1.24) but were less likely to experience post-neonatal death (HR=0.68, CI: 0.64, 0.76) as compared to twins of black mothers. We found an interaction between maternal age and weekday of birth. Twins born on weekends to teenage mothers (age<18) had a 35% greater risk for neonatal death (HR=1.35, CI: 1.06, 1.71) while those born on weekends to older mothers did not show elevated risk for any of the mortality indices.
Conclusion: Increased risks for post-neonatal death are significantly higher amongst twins born on weekends as compared to weekdays. Further research is required to identify the detailed differences in structure and procedures that result in the disadvantage associated with weekend birth.