Table 2 Characteristics of included studies

Finnstrom et al. 2006[27]

1985–1999

SWE

births: all singletons (n = 1.538.814)

 < 500, 500–999, 1000–2499 (ref.), ≥ 2500

1) neonatal mortality

1) ≤ 27d

Friedman et al. 2016[28]

1998–2010

US

women: all hospital (n = 50.433.539)

50, 1000 (ref.), 1500, 2250

1) maternal mortality

2) maternal complications

1) failure to rescue

2) severe morbidity¹

Heller et al. 2002[29]

1990–1999

GER

births: BW > 2500 g (n = 582.655);

 ≤ 500, 501–1000, 1001–1500, > 1500 (ref.)

1) Early-neonatal death

1) ≤ 7d

Hemminki et al. 2011[30]

1991–2008

FIN

births: all (n = 474.419) + BW > 2499 g in non-UH

 < 750, 750–1499, ≥ 1500, UH (ref.)

1) perinatal mortality

2) CS

1) ≤ 7d

Joyce et al. 2004[31]

1994–1996

UK

births: all (n = 540.834)

N/A: Volume entered the analysis as continuous variable

1) stand. stillbirth rates

2) stand. neonatal mortality

1) > 24 wk GA

2) ≤ 28d

Karalis et al. 2016[32]

2005–2009

FIN

births: low risk² (n = 276.066)

births: ≤ 999, 1000–1999, ≥ 2000, UH (ref.)

1) stillbirths

2) early neonatal death

1) Intrapartum: undefined

2) undefined

Moster et al. 2001[33]

1967–1996

NO

births: all (n = 1.650.852)

 ≤ 100, 101–500, 501–1000, 1001–2000, 2001–3000, > 3000 (ref.)

1) neonatal mortality

1) ≤ 28d

Pyykonen et al. 2014[34]

2006–2010

FIN

women: all³ (n = 290.288) + low risk⁴ (n = 276.287)

 < 1000, 1000–2999 (ref.), < 3000

1) perinatal mortality

2) neonatal mortality

3) early neonatal mortality

4) stillbirths

1) stillbirth + death ≤ 7d

2) ≤ 28d

3) ≤ 7d

4) ≥ 22wk GA

Snowden et al. 2012[35]

2006

US

women: all (n = 527.617), low risk⁵

Urban: ≤ 50–1199 (ref.), 1200–2399, 2400–3599; ≥ 3600

Rural: 50–599 (ref.) 600–1699; ≥ 1700

1) neonatal mortality

1) undefined

Tracy et al. 2006[36]

1999–2001

AUS

births: low risk/ term⁶ (n = 331.147)

 < 100, 100–500, 501–1000, 1001–2000, > 2001 (ref.)

1) neonatal mortality

2) CS (labour)

3) Overall CS

1) ≤ 28d

de Graaf et al. 2010[38]

2000–2006

NEL

women: singleton (n = 655.961)

 < 750, 750–999, 1000–1249, 1250–1499, 1500–1749, ≥ 1750 (ref.)

1) perinatal mortality

2) neonatal complications

1) ≤ 7d

2) Perinatal adverse outcome⁷

Restrepo et al. 2018[39]

2012

US

births: live 20–44 wk GA (n = 32.140)

N/A: Volume entered the analysis as continu-ous variable

1) neonatal mortality

1) ≤ 28d

Aubrey-Brassler et al. 2019[37]

2006–2009

CA

women: all (n = 820.761)/

births: all (n = 827.504)

No services usually; 1–49; 50–99; 100–199; 200–499; 500–999; 1000–2499, > 2500 (ref.)

1) perinatal mortality

2) maternal complications

1) Death […]⁸

2) Maternal Morbidity & Mortality⁹