Study design and population
The CPWCS population was conveniently sampled from pregnant women who received early pregnancy evaluations within a certain month from July 2017 to November 2018 in 14 maternal and child healthcare hospitals and 10 academic hospitals located in 15 provinces of China (Supplementary Fig. 1). All the 24 hospitals were public hospitals, and the cost of perinatal health care was largely covered by the government maternity insurance program. The inclusion criteria were as follows: (1) age 16 years or above, (2) pregnancy 12 weeks, as estimated based on the last menstrual period; (3) permanent resident of the study recruitment district; (4) regular antenatal inspection with the intention of delivering in the study recruitment hospital; and (5) capable of online completion of the PA assessment. The exclusion criteria were as follows: (1) serious chronic diseases, including hypertension, diabetes, heart disease, renal disease, or other diseases that would restrict PA during pregnancy; and (2) multiple pregnancy. Written informed consent was obtained from all participants, and the study was approved by the ethics review committee of Peking Union Medical College (HS-1345).
Data on population characteristics were obtained at the initial recruitment clinic visit in early pregnancy. The PA level was assessed twice, with the first assessment conducted in early pregnancy at the recruitment clinic visit and the second conducted in mid-to-late pregnancy at a prenatal clinic visit after 24 weeks of gestation.
Among 4750 women meeting the inclusion criteria, 102 were excluded due to serious chronic diseases and 32 due to multiple gestation. A total of 1,994 declined to participate. Fifty participants could not recall their PA over the previous 7 days at the first PA assessment. A total of 2572 women completed the first PA assessment in early pregnancy. Seventy-five had a miscarriage or pregnancy termination between the two assessments. Twelve participants could not recall PA at the second PA assessment. A total of 2485 women with both PA information in early and mid-to-late pregnancy were finally included in the data analysis of the present study (Fig. 1).
Data collection procedures
Population characteristics
Population characteristics that are biologically plausibility or historically reported to be associated with PA were considered as correlates investigated in our study. Demographic, pregnancy and health characteristics were obtained at the recruitment clinic visit. Demographic characteristics included age, residential region, ethnicity, educational level, annual household income and occupation. Residential regions were categorized into East, Central and West China according to the Chinese Health Statistics Yearbook. Pregnancy characteristics included parity and pregnancy plan. Pregnancy was categorized into planned and unplanned according to whether it was planned. Health characteristics included prepregnancy BMI and history of smoking or drinking. The prepregnancy 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) [31, 32]. Smoking or drinking any type of alcohol over the previous 30 days when being surveyed was defined as a history of smoking or drinking.
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 [33, 34]. 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 [35]. 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 [35]. 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 [36].
According to the IPAQ-SF, TEE on PA ≥600 MET min/week is defined as “moderate level” [36], and WHO recommends a minimum of 600 MET min/week PA to realize a health benefit [17]. 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.
Statistical analysis
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. Multivariable logistic regression was used to calculate the odds ratios (OR) and 95% confidence intervals (CI) 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.