3.3 DAG
DAG (Figure S1) was plotted according to our data analysis and previous researches[18, 19]. As it showed, for the relationship between PTB and exercise during pregnancy, the confounding factors may include age, parity, occupation, and family income. Pregnancy complications was a potential mediator, and it was not adjusted in multivariate analysis[20].
3. 4 The effect of exercise during pregnancy on PTB
Among women who did not exercise during pregnancy, the PTB rate was 5.37%. Among women who exercise during pregnancy, the PTB rate was 3.97%. Exercise during pregnancy was associated with 27% (OR = 0.73, 95% CI : 0.57-0.94) lower odds of PTB. After adjusting for the covariates, exercise during pregnancy was associated with 26% (OR = 0.74, 95% CI : 0.58-0.95) lower odds of PTB. For time spent on exercise per week, the moderate group (2.5-7h/week) was less likely to have PTB, the adjusted OR (95%CI ) was 0.72 (0.55-0.94). For energy expenditure per week, the moderate group (11.25-31.5 MET.h/week) was less likely to have PTB, the adjustedOR (95%CI ) was 0.75 (0.58-0.96) (Table 2).
For each trimester (Figure 1), according to the time spent on exercise, using the none and low group (0-2.4h/week) as a reference, different trimesters had different results. During the 1st trimester, the moderate group (2.5-7h/week) was less likely to have PTB, the adjustedOR (95%CI ) was 0.77 (0.59-0.99). During the 2nd trimester, the moderate group was less likely to have PTB, the adjusted OR s (95%CI ) were 0.74 (0.57-0.96). During the 3rd trimester, the moderate group and the high group (>7h/week) were less likely to have PTB, the adjusted OR s (95%CI ) were 0.74 (0.56-0.96) and 0.65 (0.44-0.94). According to the energy expenditure (supplementary Table 2), during the 3rd trimester, the moderate group (>7h/week) was less likely to have PTB, the adjustedOR s (95%CI ) were 0.74 (0.57-0.95). More details were provided in supplementary material Table S2 and Table S3.