Table 3 The results of univariate logistic analyses for the selection of formula feeding presented as birth year adjusted odds ratios and 95% confidence intervals

Gestational age

 <37 weeks (reference: ≥37 weeks)

1.74 (1.38–2.19)

1.67 (1.33–2.10)

1.27 (1.05–1.55)

1.25 (1.03–1.52)

1.38 (1.13–1.69)

1.34 (1.10–1.64)

Birthweight

 <2,000 g (reference: ≥2,000 g)

1.50 (1.11–2.01)

1.68 (1.29–2.18)

1.07 (0.82–1.38)

1.14 (0.91–1.43)

1.01 (0.78–1.31)

1.09 (0.86–1.37)

Maternal age at twin birth

 ≥35 years (reference: <35 years)

1.44 (1.05–1.98)

1.46 (1.06–2.00)

1.28 (0.97–1.68)

1.35 (1.03–1.77)

0.99 (0.75–1.30)

1.06 (0.80–1.40)

Birth injury

 Yes (reference: no)

0.93 (0.75–1.16)

0.89 (0.72–1.11)

0.94 (0.78–1.12)

0.87 (0.73–1.05)

1.05 (0.87–1.26)

0.95 (0.78–1.14)

Sex

 Male (reference: female)

0.78 (0.63–0.97)

0.91 (0.74–1.13)

0.96 (0.80–1.14)

1.05 (0.88–1.26)

1.07 (0.89–1.29)

1.11 (0.92–1.33)

Parity

 Primipara (reference: multipara)

1.15 (0.92–1.44)

1.12 (0.90–1.40)

0.98 (0.82–1.18)

0.95 (0.79–1.14)

1.12 (0.93–1.36)

1.09 (0.90–1.32)

Zygosity

 Monozygotic (reference: dizygotic)

0.89 (0.70–1.12)

0.91 (0.72–1.15)

1.10 (0.91–1.34)

1.11 (0.91–1.35)

1.25 (1.02–1.52)

1.17 (0.96–1.43)

Presentation

 Non-vertex (reference: vertex)

0.92 (0.67–1.26)

1.30 (1.03–1.65)

1.02 (0.79–1.32)

1.11 (0.91–1.35)

1.25 (0.95–1.63)

1.08 (0.88–1.32)

Placentation

 Monochorionic (reference: dichorionic)

0.82 (0.62–1.09)

0.84 (0.64–1.12)

0.87 (0.69–1.10)

0.87 (0.69–1.10)

0.98 (0.78–1.23)

0.93 (0.74–1.17)

Caesarean section

 Yes (reference: no)

1.20 (0.94–1.54)

1.27 (0.99–1.63)

1.03 (0.84–1.26)

1.07 (0.88–1.31)

1.11 (0.90–1.36)

1.12 (0.91–1.37)

Fertility treatment

 Yes (reference: no)

1.32 (0.95–1.84)

1.29 (0.92–1.79)

0.96 (0.74–1.25)

0.90 (0.69–1.17)

1.27 (0.98–1.65)

1.27 (0.98–1.65)

Use of incubator

 Yes (reference: no)

1.49 (1.20–1.86)

1.89 (1.51–2.36)

1.20 (1.00–1.44)

1.33 (1.11–1.60)

1.12 (0.93–1.35)

1.23 (1.02–1.49)

Previous abortion

 Yes (reference: no)

1.02 (0.78–1.32)

0.95 (0.73–1.24)

1.13 (0.91–1.40)

1.06 (0.85–1.32)

0.99 (0.79–1.23)

0.94 (0.75–1.18)

Neonatal asphyxia

 Severe or mild (reference: no)

1.21 (0.91–1.61)

1.54 (1.20–1.98)

0.92 (0.72–1.17)

1.19 (0.95–1.49)

0.95 (0.74–1.23)

1.10 (0.87–1.39)

Group

 School applicants (reference: maternal associations)

0.85 (0.57–1.27)

0.77 (0.51–1.16)

1.21 (0.91–1.60)

1.19 (0.90–1.58)

1.06 (0.81–1.39)

1.00 (0.76–1.32)