A total of 663 citations were found. One additional relevant reference was found through manual search of the reference list in the remaining papers. Of these, 584 records were excluded by title or abstract. Of the remaining 80 studies, 32 references (9 qualitative systematic reviews and 23 meta-analyses) were included for the purpose of the review (See Additional file 1: Figure S1 and Table S1 for the details on the excluded studies).

1 1 2 Risk factor [ref], year No. of studies Study design Estimates Precision Consistency Directness Publication bias Advanced parental age [23], 2017 • Highest paternal age category 20 4 cohort; 16 case-control + + − + Absent • Highest maternal age category 19 4 cohort; 15 case-control + + − + Absent Labor [25], 2011 • Prolonged labor 9 NA +/− − − + Absent • Induced or augmented labor 8 NA +/− − − + Absent • Precipitous labor 5 NA +/− − − + Absent • Premature rupture of membranes 7 NA +/− − + + Absent Delivery options [25], 2011 • Cesarean section [29], 2015 21 6 Cohort; 15 case-control + + − + Absent • Emergency cesarean 4 NA +/− − − + Absent • Elective cesarean 2 NA +/− − − + Absent • Delivery anesthesia 7 NA +/− − + + Absent • General anesthesia 3 NA +/− − + + Absent • Assisted vaginal delivery 14 NA +/− + + + Absent • Forceps 7 NA +/− − + + Absent • Vacuum extraction 2 NA −/+ − − + Absent Conditions at birth [25], 2011 • Abnormal presentation 15 NA + − − + Absent • Breech presentation 4 NA + − + + Absent • Cord complications 14 NA + − + + Absent • Fetal distress 4 NA + − + + Absent • Birth injury or trauma 6 NA ++ − + + Absent • Twins or multiple birth 10 NA + − − + Absent Maternal hemorrhage [25], 2011 4 NA ++ − + + Absent Timing of birth [25], 2011 • January through March 4 NA +/− + + + Absent • April through June 4 NA +/− − − + Absent • July through September 4 NA +/− + + + Absent • October through December 4 NA −/+ − − + Absentb • Fall 3 NA −/+ − − + Absent • Winter 3 NA −/+ − − + Absent • Spring 3 NA −/+ − − + Absent • Summer 3 NA + + + + Absent Birth spacing (ref ≥36 m) [30], 2016 7 3 cohort; 3 cross-sectional; 1 case-control • <12 months 5 NA + − − + NC • 12–23 months 5 NA +/− − − + NC • 24–35 months 5 NA −/+ − − + NC Birth spacing (ref 24–59 m) [30], 2016 • <12 months 4 NA + − − − NC • 12–23 months 4 NA + − − − NC • >60 months 4 NA + − − − NC Gestational age [25], 2011 • Postterm 14 NA +/− − − + Absent • Preterm 17 NA +/− − − + Absent • >4 weeks preterm 2 NA +/− − + + Absent Birth weight [25], 2011 • Birth weight <2500 g 15 NA + − − + Absent • Birth weight <2000 g 2 NA +/− − − + Absent • Birth weight <1500 g 3 NA ++ − + + Absent • Birth weight >4000 g 6 NA +/− − + + Absentb Neonatal clinical and medical conditions [25], 2011 • Meconium aspirated 3 NA +++ − + + Absentb • Neonatal infection 2 NA −/+ − + + Absent • Elevated temperature 2 NA +/− − − + Absent • ABO or Rh incompatible 5 NA ++ − + + Absent • Hyperbilirubinemia or jaundice 14 NA +/− − − + Absent • Jaundice 4 NA +/− − + + Absent • Hyperbilirubinemia 6 NA + − − + Absent • Phototherapy 2 NA +/− − − + Absent • Medical intervention in the first month 7 NA +/− − − + Absent Impaired gas exchange [28], 2016 • Acidosis at birth 2 2 population-based + + + + Absent • Apgar at 1 min <6 3 3 population-based ++ − + + Absent • Apgar at 1 min <7 3 3 population-based + + + + Absent • Apgar at 5 min <7 6 6 population-based + − + + Absent • Apgar at 5 min <8 3 2 population-based; 1 clinic-based ++ − + + Absent • Apgar at 5 min <9 2 1 population-based; 1 clinic-based +/− − + + Absent • Apnea/delayed crying 5 3 population