Table 4 Risk of preterm births in vitamin D deficient mothers after adjusting for co-variates

Vitamin D concentrations

 Not deficient (upper three quartiles)

Ref

Ref

 Deficient (lowest quartiles)

1.49 (1.08–2.05)

1.53 (1.10–2.12)

Sex of the baby

 Boy

Ref

 Girl

0.94 (0.71–1.25)

Mother’s age in years

 < 30

1.17 (0.75–1.83)

 ≥ 30

Ref

Mother’s education

 0–5 years

0.92 (0.69–1.22)

 > 5 years

Ref

Mother’s height

 < 145 cm

1.81 (1.29–2.54)

1.81 (1.27–2.57)

 ≥ 145 cm

Ref

Ref

Parity

 0/Primi

1.53 (1.12–2.10)

1.55 (1.12–2.12)

 1 to 3

Ref

Ref

 ≥ 4

1.33 (0.84–2.09)

1.30 (0.82–2.08)

Place of delivery

 Home

1.15 (0.83–1.58)

 Facility

Ref

Skilled delivery assistance

 Skilled birth attendant

Ref

 Untrained

1.18 (0.44–3.14)

Taking iron tablets during pregnancy

 No

Ref

 Yes

1.70 (1.20–2.40)

1.66 (1.17–2.37)

Tobacco consumption during pregnancy

 No

Ref

 Yes

1.27 (0.88–1.83)

Passive or indirect smoking during pregnancy

 No

Ref

Ref

 Yes

1.47 (1.02–2.13)

1.60 (1.09–2.34)

Chewing betel leaf during pregnancy

 No

Ref

 Yes

0.96 (0.72–1.28)

Religion

 Muslim

Ref

 Other's

0.89 (0.58–1.38)

Husband’s education

 0–5 years

1.12 (0.81–1.53)

 > 5 years

Ref

Crowding index

 ≤ 2

Ref

 > 2

0.97 (0.71–1.33)

Household wealth quintiles

 Poorest

0.96 (0.67–1.36)

 Middle

1.03 (0.73–1.46)

 Richest

Ref