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\begin{document}

\title{The effect of exercise during pregnancy on the risk of preterm birth: A
retrospective study}



\author[1]{Shouxin Bai}%
\author[1]{Yang Xiao}%
\author[1]{Shanqian Lin}%
\author[1]{Shuang Du}%
\author[1]{Zhiping Wang}%
\affil[1]{Affiliation not available}%


\vspace{-1em}



  
  \date{May 17, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Objective: To examine the association between exercise during pregnancy
and preterm birth (PTB). Design: Retrospective cohort analysis. Setting:
Jinan, China. Population: Singleton live births from December 2018 to
December 2019. Methods: Questionnaires contained items about physical
exercise (frequency, time, and primary exercise patterns) during
pregnancy and each trimester. Adjusted odds ratios (OR) were estimated
using logistic regression. Variable selection for the multivariate
models was guided by the directed acyclic graph. The median effect was
analyzed by the sequential test. Main Outcome Measures: PTB. Results:
The prevalence of PTB in this study was 4.38\% (285/6501). The adjusted
OR (95\% CI) for the risk of PTB related to exercise during pregnancy
was 0.74 (0.58-0.95). During the 1st and 2nd trimesters, the ORs (95\%
CI) for 2.5 to 7 hours of exercise per week were 0.77 (0.59-0.99) and
0.74 (0.57-0.96). During the 3rd trimester, the ORs (95\% CI) for 2.5 to
7 hours and more than 7 hours of exercise per week were 0.74 (0.56-0.96)
and 0.65 (0.44-0.94). After stratifying the subjects, the association
was only found among subjects without pregnancy complications. Pregnancy
complications partially mediated (52.40\%) the relationship between
exercise during pregnancy and PTB. Conclusions: Exercise during
pregnancy was a protective factor of PTB for women without pregnancy
complications. 2.5 to 7 hours of exercise (like walking) per week may be
appropriate in three trimesters of pregnancy, and the time could be
extended in the 3rd trimester. Keywords: Preterm birth; Exercise;
Pregnancy; Trimester; Mediation analysis%
\end{abstract}%



\sloppy


\textbf{The effect of exercise during pregnancy on the risk of preterm
birth: A retrospective study}

Shuoxin Bai \textsuperscript{1\#}, Yang Xiao \textsuperscript{2\#},
Shaoqian Lin \textsuperscript{3}, Shuang Du \textsuperscript{2}, Zhiping
Wang \textsuperscript{2*}

\emph{\textsuperscript{1} Department of Epidemiology, School of Public
Health, Cheeloo College of Medicine, Shandong University, Jinan,
Shandong, P.R. China;}

\emph{\textsuperscript{2}Department of Occupational and Environmental
Health, School of Public Health, Cheeloo College of Medicine, Shandong
University, Jinan, Shandong, P.R. China.}

\emph{\textsuperscript{3} Shandong Jinan Ecological Environmental
Monitoring Center, Jinan, Shandong, P.R. China;}

\emph{\# These authors contributed equally to this study.}

\emph{\textsuperscript{*} Correspondence:}

\emph{Address: School of Public Health, Cheeloo College of Medicine,
Shandong University, 44 Wenhuaxi Road. Jinan, Shandong, 250012P.R.
China.}

\emph{E-mail: zhipingw@sdu.edu.cn}

\emph{Funding} :

Funded by the National Natural Science Foundation of China (81773386).



\textbf{Abstract}

Objective: To examine the association between exercise during pregnancy
and preterm birth (PTB).

Design : Retrospective cohort analysis.

Setting : Jinan, China.

Population : Singleton live births from December 2018 to December 2019.

\emph{Methods:} Questionnaires contained items about physical exercise
(frequency, time, and primary exercise patterns) during pregnancy and
each trimester. Adjusted odds ratios (\emph{OR} ) were estimated using
logistic regression. Variable selection for the multivariate models was
guided by the directed acyclic graph. The median effect was analyzed by
the sequential test.

Main Outcome Measures : PTB.

Results: The prevalence of PTB in this study was 4.38\% (285/6501). The
adjusted \emph{OR} (95\% \emph{CI} ) for the risk of PTB related to
exercise during pregnancy was 0.74 (0.58-0.95). During the 1st and 2nd
trimesters, the \emph{OR} s (95\% \emph{CI} ) for 2.5 to 7 hours of
exercise per week were 0.77 (0.59-0.99) and 0.74 (0.57-0.96). During the
3rd trimester, the \emph{OR} s (95\% \emph{CI} ) for 2.5 to 7 hours and
more than 7 hours of exercise per week were 0.74 (0.56-0.96) and 0.65
(0.44-0.94). After stratifying the subjects, the association was only
found among subjects without pregnancy complications. Pregnancy
complications partially mediated (52.40\%) the relationship between
exercise during pregnancy and PTB.

Conclusions: Exercise during pregnancy was a protective factor of PTB
for women without pregnancy complications. 2.5 to 7 hours of exercise
(like walking) per week may be appropriate in three trimesters of
pregnancy, and the time could be extended in the 3rd trimester.

\textbf{Keywords} : Preterm birth; Exercise; Pregnancy; Trimester;
Mediation analysis

\textbf{Tweetable abstract:} Women who exercised during pregnancy were
at decreased risk of PTB. 2.5 to 7 hours per week may be appropriate in
three trimesters of pregnancy, and the time could be extended in the 3rd
trimester.



\textbf{1. Introduction}

Preterm birth (PTB) was the main cause of death in children younger than
5 years of age globally in 2016, accounting for approximately 35\% of
deaths among newborn babies. China had the second-highest rate of PTB in
the world in 2014\textsuperscript{{[}1{]}}. PTB leads a huge burden of
disease, and focusing on women's lifestyles during pregnancy can provide
evidence for preventing PTB.

Physical exercise is a kind of planned, organized, and repetitive
physical activity aimed at promoting a healthy body. Exercise during
pregnancy refers to any physical activity during
pregnancy\textsuperscript{{[}2{]}}. Some studies showed that both
sedentary behavior and high-intensity exercise during pregnancy can
increase the risk of premature rupture of membranes, and
moderate-intensity exercise can improve adverse pregnancy
outcomes\textsuperscript{{[}3-5{]}}.

Although there were many studies on the effect of exercise during
pregnancy on PTB, of which the conclusions were inconsistent. Studies
from different regions have not found any link between exercise during
pregnancy and PTB\textsuperscript{{[}6, 7{]}}. In a cohort study in
Brazil, the researchers used low physical activity during pregnancy as a
reference, and high or moderate physical activity did not affect the
incidence of PTB\textsuperscript{{[}6{]}}. A birth cohort in Japan
showed that compared with mothers with moderate levels of exercise,
lower levels increased the risk of preterm
delivery\textsuperscript{{[}3{]}}. Since people in different regions
have different exercise habits, more researches remain to be done.
Furthermore, few previous studies have looked at each trimester
separately. As the internal physiological environment of a woman is a
constantly changing process during pregnancy, more researches are needed
to focus on the relationship between exercise during each trimester and
PTB.

Some countries have set standards for exercise during pregnancy. For
example, the American Congress of Obstetricians and Gynecologists and
the Society of Obstetricians and Gynecologists of Canada and the
Canadian Society of Exercise Physiology had some detailed
recommendations for pregnant women\textsuperscript{{[}8-9{]}}. These
areas have high economic levels, and there are no guidelines for
exercise during pregnancy in some developing countries. The latest
physical activity guidelines for Chinese do not include a part specially
for pregnant woman\textsuperscript{{[}10{]}}. It is necessary to make
some appropriate guidelines for women in developing countries to have
scientific physical exercise during pregnancy.

Therefore, based on the data from a birth cohort in Jinan, China, we
aimed to examine the association between exercise during pregnancy and
PTB, which could provide the basis for the establishment of exercise
standards during pregnancy in China.

\textbf{2. Materials and Methods}

2.1 Study population and design

The present study was based on the baseline study of the birth cohort in
Jinan, the capital city of Shandong Province in China, from December
2018 to December 2019. Data were collected in 15 community vaccination
clinics which were selected from 116 community vaccination clinics in
the study area. Women who met the inclusion criteria were investigated
when their newborns were filed for outpatient vaccination.

Eligibility criteria included: (1) the infant was delivered without
congenital disease; (2) lived in Jinan during pregnancy; (3) does not
have mental diseases or other diseases that affect normal communication.
Only singleton live births were included.

A self-designed questionnaire for basic information of infants and
mothers was used for a one-to-one face-to-face questionnaire survey. The
information about infants includes birth outcomes, newborn birth weight,
and so on, the maternal information including her general demographic
characteristics, family economic conditions, last menstrual period and
gestational age, physical exercise during pregnancy (including the
frequency, time, and primary exercise patterns), and others included
maternal tobacco exposure, work, and nutrition during pregnancy.

The study was approved by the Ethics Committee of Preventive Medicine of
Shandong University (Approved number: 20170315), and all participants
gave their informed consent.

2.2 Outcome assessment

The outcome is PTB, which refers to the birth of a newborn at or before
the end of the last day of the 37th week after the mother's last
menstrual period\textsuperscript{{[}11{]}}. Full-term birth means the
birth during 37 to 42 weeks of gestation. In our study, last menstrual
period of women was determined by ultrasound examination at the birth
examination.

2.3 Exposure assessment

The interview questionnaire contained items about physical exercise
during pregnancy and each trimester, refers to the 1st trimester (0 to
12 weeks of gestation), the 2nd trimester (13 to 28 weeks of gestation),
and the 3rd trimester (after 28 weeks of gestation). There were some
questions about exercise on the questionnaire: 1) ``Have you exercised
during pregnancy? The exercise means any planned, organized, and
repetitive physical activity.'' 2) ``What is your primary exercise
patterns (walking, brisk walking, yoga/Pilates/maternity gymnastics,
swimming, squat and others) during pregnancy?'' For each trimester, take
the 1st trimester as an example, 3) ``How many times do you exercise per
week during the 1st trimester?'' 4) ``How many hours do you exercise at
a time?''.

Since Chinese pregnancy health guidelines do not involve exercise during
pregnancy, the subjects' activity was grouped according to the previous
studies and existing guidelines\textsuperscript{{[}8-9,12{]}}. Time
spent on exercise were grouped as none and low (0-2.49h/week), moderate
(2.5-7h/week), and high
(\textgreater{}7h/week)\textsuperscript{{[}13{]}}. One study defined
``moderate exercise'' as that requiring 4.5 METs (Metabolic Equivalent
of Energy)\textsuperscript{{[}14{]}}. So, energy expenditures were
grouped as none and low (0-11.24 MET.h/week), moderate (11.25-31.5
MET.h/week), and high (\textgreater{}31.5 MET.h/week). The calculation
of energy expenditure was based on the previous
criterion\textsuperscript{{[}14{]}}.

\textbf{2.4 Covariates}

Four types of factors were considered to be covariates. Mother's
sociodemographic characteristics included maternal age (less than 25,
25-30, and more than 30 years old), occupation (housewife/others), and
education (less than or equal to 9, 10-12, and more than 12
years)\textsuperscript{{[}15{]}}. Pregnancy-related characteristics
consisted of parity (primiparous/multiparous), delivery mode (natural
birth/cesarean), pregnancy hypertension (yes/no), delivery complications
(yes/no), work during pregnancy (yes/no), and tobacco exposure during
pregnancy (yes/no)\textsuperscript{{[}15, 16{]}}. Pregnancy
complications included gestational hypertension (HDP), amniotic fluid
embolism, uterine rupture, abnormal umbilical cord, and so on. Work
during pregnancy (no) meant pregnant women didn't work during the whole
pregnancy. Both active and passive smoking were classified as maternal
tobacco exposure due to the low number of active smokers, passive
smoking is defined as more than 30 minutes of tobacco exposure per week.
In addition, we also selected infant gender (male/female) and family
income (less than 6000 yuan/more than or equal to 6000
yuan)\textsuperscript{{[}15{]}}.

\textbf{2.5 Statistical analyses}

Categorical variables were compared by using the Chi-square test. For
continuous variables, normality was tested by the Kolmogorov-Smirnov
test. The independent sample \emph{t} -test was used for normal data,
the Wilcoxon-Mann-Whitney test was used for nonnormal data. To assess
the relationship between exercise during pregnancy and PTB risk,
Multiple logistic regression was used to calculate odds ratios
(\emph{ORs} ) and 95\% of the corresponding confidence intervals
(95\%\emph{CIs} ). Variable selection for the multiple models was guided
by the directed acyclic graph (DAG). The DAG was plotted to recognize
potential causality, mediation, or confounding
factors\textsuperscript{{[}17{]}}. Furthermore, women's exercise
activity during pregnancy was grouped by time spent and energy
expenditure, and the none and low group was used as a reference. The
effect of each trimester was observed separately. For subgroup analysis,
the subjects were stratified by whether they had pregnancy complications
or not. The median effect was analyzed by sequential test.

Statistical analyses were performed using the statistical computing
environment R 4.0.5. \emph{P-value} of \textless{}0.05 was considered
statistically significant.

\textbf{3. Results}

3.1 The characteristics of the study population

A total of 6640 pregnant women were recruited, we excluded
139multiplets. The final sample size for the study was 6501 women with
singleton live births, included 285 PTBs and 6216 full-term births, the
preterm rate was 4.38\%.

Compared to women with full-term birth, mothers who experienced PTB
tended to be older, be housewife, have more than one parity, be
cesarean, have pregnancy complications, and lower family income (Table
1).

3.2 The characteristics of exercise during each trimester

During the 3rd trimester and throughout pregnancy, the distribution of
time spent was statistically different between the preterm and full-term
women (\emph{P} =0.005; \emph{P} =0.045). The distribution of energy
expenditure was statistically different during the 3rd
trimester(\emph{P} =0.007). More details were provided in supplementary
material Table S1. Besides, for the patterns of exercise, most subjects
(90.64\%) chose the option of walking.

3.3 DAG

DAG (Figure S1) was plotted according to our data analysis and previous
researches\textsuperscript{{[}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\textsuperscript{{[}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\% (\emph{OR} =
0.73, 95\% \emph{CI} : 0.57-0.94) lower odds of PTB. After adjusting for
the covariates, exercise during pregnancy was associated with 26\%
(\emph{OR} = 0.74, 95\% \emph{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 \emph{OR} (95\%\emph{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
adjusted\emph{OR} (95\%\emph{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 adjusted\emph{OR}
(95\%\emph{CI} ) was 0.77 (0.59-0.99). During the 2nd trimester, the
moderate group was less likely to have PTB, the adjusted \emph{OR} s
(95\%\emph{CI} ) were 0.74 (0.57-0.96). During the 3rd trimester, the
moderate group and the high group (\textgreater{}7h/week) were less
likely to have PTB, the adjusted \emph{OR} s (95\%\emph{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
(\textgreater{}7h/week) was less likely to have PTB, the
adjusted\emph{OR} s (95\%\emph{CI} ) were 0.74 (0.57-0.95). More details
were provided in supplementary material Table S2 and Table S3.

3.5 The results of subgroup analysis

Among subjects with pregnancy complications, PTB rates were high
regardless of whether they exercised or not, and the adjusted
\emph{OR}(95\%\emph{CI} ) for the risk of PTB related to exercise during
pregnancy was 0.87 (0.45-1.74), which was not significant. Among
subjects without pregnancy complications, exercise during pregnancy was
associated with PTB, and the adjusted \emph{OR} (95\%\emph{CI} ) was
0.72 (0.55-0.95) (Table 3). However, after interaction analysis, no
statistically significant interaction terms were found.

3.6 The mediating effect of pregnancy complications on the relationship
between PTB and exercise during pregnancy

Pregnancy complications partially mediated the relationship between
exercise and PTB, and the mediating effect accounted for 52.40\% of the
total effect. More details for the results of mediating analysis were
provided in supplementary material Table S4.

\textbf{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\% (\emph{OR} = 0.74, 95\% \emph{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 (\emph{OR} = 0.82, 95\% \emph{CI} :
0.76-0.88)\textsuperscript{{[}21{]}}. A study in Southern California
reported that both moderate exercise (\emph{OR} = 0.90, 95\% \emph{CI} :
0.84-0.96) and vigorous exercise (\emph{OR} = 0.67, 95\% \emph{CI} :
0.46-0.98) during pregnancy were associated with lower risk of
PTB\textsuperscript{{[}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\textsuperscript{{[}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 \emph{OR} s ranged from 0.43
to 0.65\textsuperscript{{[}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\% (\emph{OR} =0.80, 95\% \emph{CI} : 0.68--0.92),
30\% (\emph{OR} = 0.70, 95\% \emph{CI} : 0.60--0.82), and 32\%
(\emph{OR}= 0.68, 95\% \emph{CI} : 0.59--0.78) lower odds of PTB,
respectively\textsuperscript{{[}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\textsuperscript{{[}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\textsuperscript{{[}25{]}}, and proper exercise helps pregnant
women maintain a reasonable weight\textsuperscript{{[}26{]}}. Obesity
alters levels of related inflammatory
cytokines\textsuperscript{{[}27{]}}, and elevated levels of inflammatory
cytokines can stimulate increased levels of oxytocin and lead to
PTB\textsuperscript{{[}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\textsuperscript{{[}29{]}}, which can reduce the risk of
PTB\textsuperscript{{[}30{]}}.

Until now, the mechanism between exercise and PTB has several
hypotheses. Firstly, placental hypoplasia is one of the important causes
of PTB\textsuperscript{{[}31{]}}, and there is much evidence that
exercise during pregnancy promotes placenta
development\textsuperscript{{[}32, 33{]}}. For one thing, exercise leads
to a significant increase in placenta volume during the second
trimester\textsuperscript{{[}32{]}}. For another, exercise during
pregnancy promotes placenta angiogenesis\textsuperscript{{[}33{]}}.
Additionally, exercise during pregnancy may confer a protective effect
against PTB through IL-10 mediated pathways\textsuperscript{{[}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\textsuperscript{{[}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\textsuperscript{{[}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\textsuperscript{{[}7{]}}, which is considered a risk factor for
PTB\textsuperscript{{[}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.

\textbf{5. Conclusion}

Exercise during pregnancy was a protective factor of PTB for women
without pregnancy complications. 2.5 to 7 hours of exercise (like
walking) per week may be appropriate in three trimesters of pregnancy,
and the time can be extended in the 3rd trimester. To provide the basis
for the formulation of exercise guidelines for pregnant women in China
and Asia, more researches are needed to be done.

\textbf{Acknowledgments}

Thanks to the Jinan Center for Disease Control and Prevention and the
staff of the community vaccination clinic for the investigation. We also
thank all mothers and infants for their cooperation.

\textbf{Competing interests}

All authors certify that they have no affiliations with or involvement
in any organization or entity with any financial interest or
non-financial interest in the subject matter or materials discussed in
this manuscript.

\textbf{Contribution to authorship}

SXB, YX: Study conception and design, analysis of data, drafting and
revising the article for critically important intellectual content,
final approval of the version to be published. SQL, SD: Design of
questionnaire, collection of data, drafting and revising the article.
ZPW: Study conception and design, ZPW served as senior author and
provided guidance to SXB and YX throughout the project.

\textbf{Ethics approval and consent to participate}

The study was approved by the Ethics Committee of Preventive Medicine of
Shandong University (Approved number: 20170315). Informed consent was
obtained from mothers of subjects.

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Table 1. Characteristics of the study population by PTB status\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
\textbf{Covariates} & \textbf{Preterm(\emph{n}=285)} &
\textbf{Preterm(\emph{n}=285)} & \textbf{Full-term(\emph{n=}6216)} &
\textbf{Full-term(\emph{n=}6216)} & \textbf{PTB rates(\%)} &
\selectlanguage{greek}Χ\selectlanguage{english}\textsuperscript{2} & P-value\tabularnewline
\midrule
\endhead
& n & \% & n & \% & & &\tabularnewline
\textbf{Mother} & & & & & & &\tabularnewline
Age (years) & & & & & & 11.229 & \textbf{0.004}\tabularnewline
\textless{}25 & 17 & 5.96 & 520 & 8.37 & 3.17 & &\tabularnewline
25-30 & 106 & 37.19 & 2785 & 44.80 & 3.67 & &\tabularnewline
\textgreater{}30 & 162 & 56.84 & 2911 & 46.83 & 5.27 & &\tabularnewline
Occupation & & & & & & 5.200 & \textbf{0.023}\tabularnewline
Other & 210 & 73.68 & 4940 & 79.47 & 4.08 & &\tabularnewline
Housewife & 75 & 26.32 & 1276 & 20.53 & 5.55 & &\tabularnewline
Education (years) & & & & & & 3.719 & 0.156\tabularnewline
[?]9 & 27 & 9.47 & 568 & 9.14 & 4.54 & &\tabularnewline
10-12 & 61 & 21.40 & 1064 & 17.12 & 5.42 & &\tabularnewline
\textgreater{}12 & 197 & 69.12 & 4584 & 73.75 & 4.12 & &\tabularnewline
\textbf{Pregnancy} & & & & & & &\tabularnewline
Parity & & & & & & 4.554 & \textbf{0.033}\tabularnewline
Primiparous & 122 & 42.81 & 3074 & 49.45 & 3.82 & &\tabularnewline
Multiparous & 163 & 57.19 & 3142 & 50.55 & 4.93 & &\tabularnewline
Delivery mode & & & & & & 24.499 &
\textbf{\textless{}0.001}\tabularnewline
natural birth & 114 & 40.00 & 3426 & 55.12 & 3.22 & &\tabularnewline
cesarean & 171 & 60.00 & 2790 & 44.88 & 5.78 & &\tabularnewline
Complications & & & & & & 80.40 &
\textbf{\textless{}0.001}\tabularnewline
Yes & 46 & 16.14 & 268 & 4.31 & 14.65 & &\tabularnewline
No & 239 & 83.86 & 5948 & 95.69 & 3.86 & &\tabularnewline
Work & & & & & & \textless{}0.001 & 0.997\tabularnewline
Yes & 175 & 61.40 & 3829 & 61.60 & 4.45 & &\tabularnewline
No & 110 & 38.60 & 2387 & 38.40 & 4.41 & &\tabularnewline
Tobacco exposure & & & & & & 0.0688 & 0.793\tabularnewline
Yes & 41 & 14.39 & 941 & 15.14 & 4.18 & &\tabularnewline
No & 244 & 85.61 & 5275 & 84.86 & 4.42 & &\tabularnewline
\textbf{Infant} & & & & & & &\tabularnewline
Gender & & & & & & 1.639 & 0.200\tabularnewline
Male & 164 & 57.54 & 3209 & 51.62 & 4.86 & &\tabularnewline
Female & 121 & 42.46 & 3007 & 48.38 & 3.87 & &\tabularnewline
\textbf{Family} & & & & & & &\tabularnewline
Income(yuan/month) & & & & & & 5.415 & \textbf{0.020}\tabularnewline
\textless{}6000 & 49 & 17.19 & 767 & 12.34 & 6.00 & &\tabularnewline
[?]6000 & 236 & 82.81 & 5449 & 87.66 & 4.15 & &\tabularnewline
\bottomrule
\end{longtable}

Table 2. Association between PTB and exercise during pregnancy\selectlanguage{english}
\begin{longtable}[]{@{}lllllllll@{}}
\toprule
\textbf{During pregnancy} & \textbf{PTB rate(\%)} & \textbf{Preterm} &
\textbf{Full-term} & \textbf{Unadjusted} & \textbf{Unadjusted} & &
\textbf{Adjusted} & \textbf{Adjusted}\tabularnewline
\midrule
\endhead
& & \textbf{(\emph{n})} & \textbf{(\emph{n})} & OR &
\textbf{95\%\emph{CI}} & & OR & \textbf{95\%\emph{CI}}\tabularnewline
Exercise & & & & & & & &\tabularnewline
No & 5.37 & 103 & 36.14 & 1.00 & & & 1.00 &\tabularnewline
Yes & 3.97 & 182 & 63.86 & \textbf{0.73*} & \textbf{0.57-0.94} & &
\textbf{0.74*} & \textbf{0.58-0.95}\tabularnewline
Time spent on exercise per week & & & & & & & &\tabularnewline
None and low & 5.28 & 112 & 39.30 & 1.00 & & & 1.00 &\tabularnewline
Moderate & 3.79 & 125 & 43.86 & \textbf{0.71*} & \textbf{0.54-0.92} & &
\textbf{0.72*} & \textbf{0.55-0.94}\tabularnewline
High & 4.44 & 48 & 16.84 & 0.83 & 0.59-1.17 & & 0.82 &
0.58-1.16\tabularnewline
Energy expenditure per week & & & & & & & &\tabularnewline
None and low & 4.99 & 145 & 50.88 & 1.00 & & & 1.00 &\tabularnewline
Moderate & 3.76 & 113 & 39.65 & \textbf{0.74*} & \textbf{0.58-0.95} & &
\textbf{0.75*} & \textbf{0.58-0.96}\tabularnewline
High & 4.58 & 27 & 9.47 & 0.91 & 0.59-1.37 & & 0.89 &
0.57-1.33\tabularnewline
Adjusted for maternal age, maternal occupation, parity, and family
income. * \emph{P} \textless{} 0.05, ** \emph{P} \textless{}0.01, ***
\emph{P} \textless{}0.001 & Adjusted for maternal age, maternal
occupation, parity, and family income. * \emph{P} \textless{} 0.05, **
\emph{P} \textless{}0.01, *** \emph{P} \textless{}0.001 & Adjusted for
maternal age, maternal occupation, parity, and family income. * \emph{P}
\textless{} 0.05, ** \emph{P} \textless{}0.01, *** \emph{P}
\textless{}0.001 & Adjusted for maternal age, maternal occupation,
parity, and family income. * \emph{P} \textless{} 0.05, ** \emph{P}
\textless{}0.01, *** \emph{P} \textless{}0.001 & Adjusted for maternal
age, maternal occupation, parity, and family income. * \emph{P}
\textless{} 0.05, ** \emph{P} \textless{}0.01, *** \emph{P}
\textless{}0.001 & Adjusted for maternal age, maternal occupation,
parity, and family income. * \emph{P} \textless{} 0.05, ** \emph{P}
\textless{}0.01, *** \emph{P} \textless{}0.001 & Adjusted for maternal
age, maternal occupation, parity, and family income. * \emph{P}
\textless{} 0.05, ** \emph{P} \textless{}0.01, *** \emph{P}
\textless{}0.001 & Adjusted for maternal age, maternal occupation,
parity, and family income. * \emph{P} \textless{} 0.05, ** \emph{P}
\textless{}0.01, *** \emph{P} \textless{}0.001 & Adjusted for maternal
age, maternal occupation, parity, and family income. * \emph{P}
\textless{} 0.05, ** \emph{P} \textless{}0.01, *** \emph{P}
\textless{}0.001\tabularnewline
\bottomrule
\end{longtable}

Table 3. Association between PTB and exercise during pregnancy in
subgroup analysis\selectlanguage{english}
\begin{longtable}[]{@{}lllllllllll@{}}
\toprule
\textbf{Complication} & \textbf{Exercise during pregnancy} &
\textbf{Preterm rates(\%)} & \textbf{Preterm} & \textbf{Full-term} & &
\textbf{Unadjusted} & \textbf{Unadjusted} & & \textbf{Adjusted} &
\textbf{Adjusted}\tabularnewline
\midrule
\endhead
& & & n & n & & OR & \textbf{95\%\emph{CI}} & & OR &
\textbf{95\%\emph{CI}}\tabularnewline
Yes & No & 14.95 & 16 & 91 & & 1.00 & & & 1.00 &\tabularnewline
& Yes & 14.49 & 30 & 177 & & 0.96 & 0.51-1.90 & & 0.87 &
0.45-1.74\tabularnewline
No & No & 4.81 & 87 & 1723 & & 1.00 & & & 1.00 &\tabularnewline
& Yes & 3.47 & 152 & 4225 & & \textbf{0.71*} & \textbf{0.55-0.94} & &
\textbf{0.72*} & \textbf{0.55-0.95}\tabularnewline
Adjusted for maternal age, maternal occupation, parity, and family
income. & Adjusted for maternal age, maternal occupation, parity, and
family income. & Adjusted for maternal age, maternal occupation, parity,
and family income. & Adjusted for maternal age, maternal occupation,
parity, and family income. & Adjusted for maternal age, maternal
occupation, parity, and family income. & Adjusted for maternal age,
maternal occupation, parity, and family income. & Adjusted for maternal
age, maternal occupation, parity, and family income. & Adjusted for
maternal age, maternal occupation, parity, and family income. & Adjusted
for maternal age, maternal occupation, parity, and family income. &
Adjusted for maternal age, maternal occupation, parity, and family
income. & Adjusted for maternal age, maternal occupation, parity, and
family income.\tabularnewline
\emph{P} \textless{} 0.05, ** \emph{P} \textless{}0.01, *** \emph{P}
\textless{}0.001 & * \emph{P} \textless{} 0.05, ** \emph{P}
\textless{}0.01, *** \emph{P} \textless{}0.001 & * \emph{P} \textless{}
0.05, ** \emph{P} \textless{}0.01, *** \emph{P} \textless{}0.001 & *
\emph{P} \textless{} 0.05, ** \emph{P} \textless{}0.01, *** \emph{P}
\textless{}0.001 & * \emph{P} \textless{} 0.05, ** \emph{P}
\textless{}0.01, *** \emph{P} \textless{}0.001 & * \emph{P} \textless{}
0.05, ** \emph{P} \textless{}0.01, *** \emph{P} \textless{}0.001 & *
\emph{P} \textless{} 0.05, ** \emph{P} \textless{}0.01, *** \emph{P}
\textless{}0.001 & * \emph{P} \textless{} 0.05, ** \emph{P}
\textless{}0.01, *** \emph{P} \textless{}0.001 & * \emph{P} \textless{}
0.05, ** \emph{P} \textless{}0.01, *** \emph{P} \textless{}0.001 & *
\emph{P} \textless{} 0.05, ** \emph{P} \textless{}0.01, *** \emph{P}
\textless{}0.001 & * \emph{P} \textless{} 0.05, ** \emph{P}
\textless{}0.01, *** \emph{P} \textless{}0.001\tabularnewline
\bottomrule
\end{longtable}

Figure 1. Association between PTB and exercise during each trimester\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Figure-1/Figure-1}

\end{center}
\end{figure}

\selectlanguage{english}
\FloatBarrier
\end{document}

