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.