Readmissions analyses typically calculate time-to-readmission relative to hospital discharge. For newborns, this definition can be challenging when comparing groups with disparate birth stays. We compare 2 approaches to calculate readmissions and examine 1 year readmissions for newborns with versus without neonatal opioid withdrawal syndrome (NOWS; mean length of stay = 17 vs 2 days).


Using birth discharge data from the Pediatric Health Information System (PHIS), we compared crude and adjusted 1 year readmissions by NOWS diagnosis using Cox regression models predicting time-to-readmission from: (1) birth discharge; and (2) birth (day-of-life), with left truncation allowing for delayed entry into the at-risk period at birth discharge.


We included N = 155 885 birth discharges (n = 1467 with NOWS). At 1 year, 10% of infants with NOWS versus 6% without had been readmitted. Readmission risk was highest within 1 week since discharge or birth for newborns without NOWS, whereas those with NOWS were at higher risk later into infancy. NOWS was associated with a higher adjusted hazard of 1 year readmissions since discharge (adjusted hazard ratio [aHR]=1.58; 95% CI: 1.20–2.08) and a higher adjusted hazard of 1 year readmissions since birth (aHR = 1.56; 95% CI: 1.21–2.03). Estimates vary by choice of index date, particularly at early time-points, converging later into infancy.


Our findings underscore the importance of methodological decisions for newborn readmissions. Although results were similar at 1 year with nearly identical adjusted hazards, approaches differed substantially through the neonatal period.

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