Sensitivity and subgroup analyses
In the sensitivity analyses, the association of pre-pregnancy BMI with the six outcomes did not substantially change with additional adjustment for the length of time from pre-pregnancy examination to last menstrual period or inclusion of women self-reported with perceived economic pressure (Table S7).
The association of pre-pregnancy BMI with risk of the five outcomes by maternal age are shown in Figure 1 and 2. Underweight was inversely associated with risk of LGA birth among those younger than 40 years (0.88, 0.85-0.91; 0.82, 0.80-0.84; 0.77,0.73-0.81; and 0.65, 0.58-0.74 among those aged 19- 24 years, 25-29 years, 30-34 years, 35- 39 years, respectively), and primary caesarean delivery among those younger than 35 years (0.89, 0.87-0.92; 0.86, 0.84-0.88; and 0.92, 0.88-0.97 among those aged 19-24 years, 25-29 years and 30-34 years, respectively), but positively associated with risk of SGA among those younger than 35 years (1.2, 1.17-1.23; 1.24, 1.21-1.27; and 1.32, 1.26-1.39 among those aged 19-24 years, 25-29 years and 30-34 years, respectively).
Overweight was inversely associated with risk of SGA birth among those aged 25-29 years (0.92, 0.88-0.96) and 30-34 year (0.88, 0.82-0.92), but positively associated with risk of PTB among those aged 35-39 years (1.22, 1.10-1.36), LGA among all the age groups (19-24 years: 1.11, 1.06-1.16; 1.17, 25-29 years: 1.17, 1.13-1.21; 30-34 years: 1.21, 1.16-1.26; 35-39 years: 1.20, 1.14-1.27 and 40-50 year:1.17, 1.02-1.34), and primary caesarean delivery among those younger than 35 years (19-24 years: 1.21, 1.17-1.25; 25-29 years: 1.21, 1.17-1.24; 30-34 years: 1.10, 1.10-1.15), .
Obesity was inversely associated with risk of SGA among those aged 19-24 years (0.89, 0.80-0.99), but positively associated with risk of PTB among those aged younger than 40 years (25-29 years: 1.15, 1.03-1.29; 30-34 years: 1.16, 1.01-1.34; and 35-39 years: 1.25, 1.03-1.52), LGA among all age groups (19-24 years: 1.22, 1.13-1.33; 25-29 years: 1.34, 1.26-1.43; 30-34 years: 1.34, 1.25-1.45; 35-39 years: 1.36, 1.23-1.51 and 40-50 year:1.44, 1.14-1.83), and primary caesarean delivery among those younger than 40 years (19-24 years: 1.52, 1.43-1.62; 25-29 years: 1.42, 1.35-1.50; 1.42, 30-34 years: 1.31-1.54, and 35-39 years: 1.31, 1.12-1.53).