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.