Study design and participants
This multicenter, retrospective cohort study of postpartum women was conducted in 14 clinical centers located in urban areas of China from June 2012 to March 2013. Details of this study have been described elsewhere [10]. Briefly, we recruited postpartum women aged 18 years or above with a gestational age of 28-42 weeks and who gave live birth during 10th-19th of the last month of every quarter in order to control for seasonal variations. Among 9,152 participants with full medical records, 308 women with pregestational diabetes (n=38), multiple gestation (n=223), pre-conception history of severe heart disease or chronic renal disease (n=48) were excluded. Overall, the present analysis was restricted to 8,844 deliveries. This study was approved by the institutional review board of Peking University First Hospital, and all participants provided written informed consent.
Data collection
Information on demographic characteristics, lifestyle behavior, medical history of pregnancy, and pregnancy outcomes were collected by using a structured questionnaire after delivery. In-person interviews were conducted to collect information on demographic characteristics such as age, education, employment and annual household income, as well as lifestyle behavior including drinking and passive smoking during pregnancy, and clinical data such as medical history and pregnancy outcomes was extracted from the medical records.
Gestational age at delivery was determined from the date of last menstrual period to the date of delivery and expressed in the week after the last menstrual period. If the date was uncertain, ultrasonography was used to determine gestational age. Weight and height at the first antenatal visit prior to the 13th gestational week, and weight at the last antenatal visit within 2 weeks before delivery or the time of delivery were extracted from the medical records. Prepregnancy BMI was calculated as the weight in kilograms divided by the square of height measured in meters, and classified into three groups according to the Chinese standard [17]: underweight (BMI <18.5 kg/m2), normal weight (18.5 kg/m2 ≤ BMI <24 kg/m2), overweight and obese (BMI ≥ 24 kg/m2). The GWG was calculated by subtracting the weight measured at the first antenatal visit from the final weight measured at the last antenatal visit or the time of delivery. All participants were divided into three groups according to GWG, defined by 25th and 75th percentile of GWG (12.0 and 19.0 kg): lower (GWG <12 kg), middle (12 kg ≤GWG <19 kg), higher (≥19 kg).
Diagnosis of GDM
According to the International Association of Diabetes and Pregnancy Study Groups (IADPSG) criteria, the diagnosis of GDM should be made when any one 75-g oral glucose tolerance test value met or exceeded 5.1 mmol/L at 0 h, 10.0 mmol/L at 1 h, 8.5 mmol/L at 2 h when performed between 24 and 28 gestational weeks [18].
Definition of adverse pregnancy outcomes
The main outcomes of the study were cesarean delivery, preterm birth, low birth weight, small-for-gestational age (SGA), macrosomia and LGA. Preterm birth was defined as all birth before 37 completed weeks or before 259 completed days since the first day of a women’s last menstrual period [19]. Low birth weight was defined as the neonatal birth weight <2500 g, and macrosomia as birth weight ≥ 4000 g, respectively. LGA and SGA were indicated by birth weight less than and greater than the 10th and 90th percentile, respectively, for the same gestational age by sex, according to the Chinese neonatal birth weight curve [20].
Statistical analysis
Demographics characteristics and pregnancy outcomes were expressed as numbers and frequency distributions for categorical variables, or median and interquartile range for continuous variables. To compare between groups, the chi-square test and Mann-Whitney U test were performed for categorical variables and skewed distributed continuous variables, respectively. Logistic regression models were conducted to estimate odds ratios (ORs) and their 95% confidence intervals (CIs) of pregnancy outcomes and GDM, and interaction between GDM and maternal age/prepregnancy BMI/GWG groups. Models were adjusted for maternal age (continuous), education (high school and below, college or graduate school), employment (unemployed, employed), annual household income (<10000 RMB, 10000-20000 RMB, <20000 RMB), drinking during pregnancy (yes, no), passive smoking during pregnancy (yes, no), prepregnancy BMI categories (underweight, normal weight, overweight/obese), parity (primiparous, multiparous), use of assisted reproductive technology (ART, yes, no), folic acid supplementation (yes, no), gestational age at delivery (continuous, except for the outcome of preterm birth) and GWG categories (lower, middle, higher). The non-GDM group was used as the reference group. Stratified analyses were performed according to maternal age groups (<35 years, ≥35 years), prepregnancy BMI categories (underweight, normal weight, overweight/obese) and GWG categories (lower, middle, higher). Analyses were carried out using SAS software version 9.2 (SAS Institute, Cary, NC). All P values are two-sided, and statistical significance was defined as P< 0.05.