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[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[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[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[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[6]. A birth cohort in Japan showed that
compared with mothers with moderate levels of exercise, lower levels
increased the risk of preterm delivery[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[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[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.