Study design and population
The CPWCS recruited pregnant women from 14 maternal and child healthcare hospitals and 10 academic hospitals located in 15 provinces of China. Seven, nine and eight hospitals were in East, Central, and West China, respectively. Each hospital has approximately 400 deliveries per month. Pregnant women who received early pregnancy evaluations at each study recruitment hospital within a certain month from July 2017 to November 2018 were screened for eligibility. The inclusion criteria were as follows: women were aged 16 years or above, with a gestational age of 5-12 weeks; were permanent residents of the study recruitment district; had the intention of delivering in the study recruitment hospital; and had agreed to participate in the CPWCS. The exclusion criteria were women with serious chronic diseases and psychosis. The CPWCS was approved by the ethics review committee of Peking Union Medical College Hospital, the leading centre (HS-1345). Written informed consent was obtained from all participants.
Among all participants in the CPWCS, only those who could recall their PA over the previous 7 days when being surveyed were eligible for our PA study. Women were excluded from the PA study if they had a multiple pregnancy (n=32), a miscarriage (n=74), or a chronic disease (n=102) including hypertension, diabetes, heart disease, renal disease, or other diseases that would restrict PA during pregnancy. A total of 2485 women with PA information in both early and mid-to-late pregnancy were finally included for analysis in our study.
Demographic, pregnancy-related and health-related data were obtained at the initial recruitment clinic visit in early pregnancy (mean=10.7 weeks of gestation). The PA level was assessed at the initial recruitment clinic visit and updated at the prenatal clinic visit in mid-to-late pregnancy (mean=32.0 weeks of gestation).
Information on age, residential region, ethnicity, educational level, annual household income, occupation, parity, pregnancy plan, height, prepregnancy weight, and history of smoking or drinking was obtained at the initial recruitment clinical visit. Residential regions were categorized into East, Central and West China according to the Chinese Health Statistics Yearbook. The prepregnancy body mass index (BMI) (kg/m2) was calculated based on the self-reported prepregnancy weight in kilograms and height in centimetres. BMI was categorized as underweight, normal weight, overweight and obese (<18.5, 18.5–23.9, ≥24, respectively) [23, 24].
Assessment of PA
PA was assessed in early and mid-to-late pregnancy using the International Physical Activity Questionnaire short form (IPAQ-SF) validated for the Chinese population [25, 26]. The IPAQ addresses three types of PA: high-intensity PA, medium-intensity PA, and walking. High-intensity PA refers to activities that require hard physical effort and that make breathing much harder than normal, such as heavy lifting, digging, or aerobics . Medium-intensity PA refers to activities that take moderate physical effort and make breathing somewhat harder than normal, such as carrying light loads, bicycling at a regular pace, or table tennis . Walking includes all walking for occupation, transportation, household, exercise and leisure. The frequency (days) and duration (minutes) of each PA over the previous seven days were investigated. Total energy expenditure (TEE) on PA per week was calculated as a total of three types of PA reported in the MET value × minutes per week. The values of 3.3, 4.0 and 8.0 were assigned to represent the MET values of walking, medium-intensity PA and high-intensity PA, respectively .
According to the IPAQ-SF, TEE on PA ≥600 MET min/week is defined as “moderate level” , and WHO recommends a minimum of 600 MET min/week PA to realize a health benefit . Therefore, we defined PA with TEE ≥600 MET min/week as sufficient PA and PA with TEE <600 MET min/week as insufficient PA. Sufficient PA in early pregnancy and insufficient PA in mid-to-late pregnancy indicated decreasing PA. Insufficient PA in early pregnancy and sufficient PA in mid-to-late pregnancy indicated increasing PA.
The population characteristics of all women included in the study are described. Categorical data are expressed as frequencies and percentages. Continuous data are expressed as means, standard deviations (SDs), medians and interquartile ranges (IQRs). TEE on PA, energy expenditure on each type of PA, and the proportion of energy expenditure on each type of PA to TEE on PA were compared between early and mid-to-late pregnancy using the Wilcoxon signed-rank test. The proportions of women with sufficient PA levels were compared between early and mid-to-late pregnancy using McNemar’s test. Univariate and multivariate logistic regression were used to calculate the unadjusted and adjusted odds ratios (ORs) and 95% confidence intervals (CIs) to address the following: (1) associations between population characteristics and sufficient PA in mid-to-late pregnancy among all women included in the study, (2) associations between population characteristics and increasing PA among the subset of women with insufficient PA levels in early pregnancy, and (3) associations between population characteristics and decreasing PA among the subset of women with sufficient PA levels in early pregnancy. P values <0.05 were considered statistically significant. SPSS 22.0 (IBM, Armonk, NY, USA) was used for statistical analysis.