Table 3 Relative risk (RR) of preterm birth in each combination of maternal educational level and mental health conditions stratified by age group, RR [95% CI]

≤23

High/intermediate

1 [ref]

1.05 [0.90;1.22]

1.15 [0.99;1.32]

Low

1.04 [0.97;1.12]

1.08 [0.95;1.24]

1.25 [1.14;1.37]

24–30

High

1 [ref]

1.08 [1.00;1.17]

1.16 [1.04;1.29]

Intermediate

1.13 [1.09;1.17]

1.22 [1.13;1.32]

1.28 [1.18;1.39]

Low

1.14 [1.06;1.21]

1.50 [1.33;1.69]

1.53 [1.39;1.68]

≥31

High

1 [ref]

1.15 [1.07;1.24]

1.31 [1.18;1.45]

Intermediate

1.25 [1.19;1.32]

1.41 [1.28;1.55]

1.56 [1.39;1.74]

Low

1.46 [1.33;1.60]

1.52 [1.27;1.83]

2.23 [1.93;2.58]