4. Discussion
In our study, the PTB rates were 3.97% and 5.37% among women who did or did not exercise during pregnancy. We found that exercise during pregnancy was associated with 26% (OR = 0.74, 95% CI : 0.58-0.95) lower odds of PTB. For the association between exercise during pregnancy and PTB, most relevant studies agree with us. One study in Denmark showed a reduced risk of PTB among the women who exercised during pregnancy (OR = 0.82, 95% CI : 0.76-0.88)[21]. A study in Southern California reported that both moderate exercise (OR = 0.90, 95% CI : 0.84-0.96) and vigorous exercise (OR = 0.67, 95% CI : 0.46-0.98) during pregnancy were associated with lower risk of PTB[22]. A meta-analysis included only randomized controlled trials (RCTs) of overweight or obese pregnant women, showed that women who had an aerobic exercise for about 30–60 min three to seven times per week had a lower percentage of PTB (RR = 0.62, 95% CI: 0.41-0.95) compared with controls[23]. And two studies in China also had similar results. Huang et al. did analyses about the relationship between maternal exercise frequency and duration during pregnancy and PTB, and the adjusted OR s ranged from 0.43 to 0.65[24]. Cai et al. reported that women who participated in physical exercise 1-2 times, 3-4 times, and over five times per week had 20% (OR =0.80, 95% CI : 0.68–0.92), 30% (OR = 0.70, 95% CI : 0.60–0.82), and 32% (OR= 0.68, 95% CI : 0.59–0.78) lower odds of PTB, respectively[15]. However, there were studies with negative results. For example, a cohort study in Brazil found no link between high or moderate physical activity and PTB[6], and this may be due to racial differences.
We further divided pregnancy into three trimesters, to observe the relationship between exercise during each trimester and PTB. The results suggested that, during the 1st and 2nd trimesters, 2.5 to 7 hours of exercise per week was associated with lower odds of PTB. During the 3rd trimester, both 2.5 to 7 hours and more than 7 hours of exercise per week were associated with lower odds of PTB. Compared to the 1st and 2nd trimesters, the 3rd trimester may require a longer period of exercise. The reason may be that pregnant women gained more weight during the 3rd trimester than during the 1st and 2nd trimesters[25], and proper exercise helps pregnant women maintain a reasonable weight[26]. Obesity alters levels of related inflammatory cytokines[27], and elevated levels of inflammatory cytokines can stimulate increased levels of oxytocin and lead to PTB[28]. Also, a longer time of relaxed exercise activities in the 3rd trimester may help pregnant women relieve tension, promote blood circulation, increase pelvic floor muscle strength[29], which can reduce the risk of PTB[30].
Until now, the mechanism between exercise and PTB has several hypotheses. Firstly, placental hypoplasia is one of the important causes of PTB[31], and there is much evidence that exercise during pregnancy promotes placenta development[32, 33]. For one thing, exercise leads to a significant increase in placenta volume during the second trimester[32]. For another, exercise during pregnancy promotes placenta angiogenesis[33]. Additionally, exercise during pregnancy may confer a protective effect against PTB through IL-10 mediated pathways[34].
After stratifying the subjects by whether they had pregnancy complications or not, the association between PTB and exercise during pregnancy only was found among subjects without pregnancy complications. People with pregnancy complications always are considered to be at high risk for PTB[18], this may account for the effect of exercise on PTB was not significant in these people. We didn’t adjust pregnancy complications in multivariate analysis, because it was a potential mediator in the causal pathway between exposure and outcome, the adjustment of it in the model may affect the estimation of the results based on previous studies[20]. And our study indicated that pregnancy complications had a partial mediating effect, which was consistent with previous studies. A meta-analysis showed that exercise during pregnancy reduces the risk of HDP[7], which is considered a risk factor for PTB[18].
There are some strengths of this study. Firstly, this study specifically analyzes the1st, 2nd, and 3rd trimester exercise and PTB. Secondly, this study provides relevant evidence for the formulation of relevant standards in China. Thirdly, we firstly found a partial mediating effect of pregnancy complications on the relationship between PTB and exercise during pregnancy.
We also have some limitations. There is recall bias due to the research method being a face-to-face questionnaire. In the multivariate analysis, we did not adjust the time spent on exercise during the 1st, 2nd, and 3rd trimester for each other, because there was collinearity between the three pregnancies. And there are some confounding factors we cannot control, such as dietary differences between regions. Moreover, we didn’t adjust gestational weight gain in our regression model, which may be an important variable.