Study design and study population

This prospective cohort study was conducted between September 7, 2020, and July 31, 2023, as part of a study exploring strategies to improve the metabolic health of postpartum women with a history of GDM. The study protocol was approved by the Vajira Institutional Review Board (approval no. 017/2563) and performed in accordance with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines. The protocol was registered with the Thai Clinical Trials Registry (registration no. TCTR20200903001).

The study population included women aged ≥ 18 years with recent GDM who delivered a live infant at the Faculty of Medicine Vajira Hospital, Bangkok, Thailand, between September 7, 2020, and January 31, 2023. Inclusion criteria were women who underwent antenatal care and GDM screening at the hospital. The exclusion criteria were pregnancy during the 6 months of study involvement, loss to follow-up, and refusal to participate.

Sample size

Given that no study has directly explored the effects of postpartum weight changes on changes in the prevalence rates of MetS and its components among women with a history of GDM, the sample size was calculated based on data from a previous study that explored the effects of weight changes between 3 and 12 months on changes in the levels of metabolic parameters in this population [17]. Based on the findings that changes in WC among women in the weight loss, weight stability, and weight gain groups were − 5.4 ± 5.1 cm, − 1.8 ± 4.6 cm, and 1.4 ± 4.6 cm, respectively, the sample size was calculated using the data on WC changes in the weight stability and weight gain groups because this yielded the largest sample. With 80% power at a two-sided significance level of 0.05, at least 99 participants were required (33 each with weight loss, weight stability, and weight gain). Allowing for a dropout rate of 25%, the total sample size required was 132 (44 in each group).

Institutional practices for GDM screening and diagnosis

The institutional practices for GDM screening and diagnosis have been described in detail in our previous work [20]. In brief, we followed the standard recommendation that all pregnant women undergo GDM screening using a 50-g glucose challenge test, followed by a 100-g oral glucose tolerance test (OGTT) using the Carpenter and Coustan criteria if the screening result was abnormal [21]. Blood samples were collected by phlebotomists and sent to the hospital laboratory. The initial management of women with GDM consists of dietary and lifestyle modifications, followed by insulin therapy if fasting or postprandial plasma glucose levels remain high despite dietary management [21].

After delivery, all women with GDM are scheduled for a postpartum checkup and T2DM screening using a 75-g, 2-h OGTT at 6 weeks postpartum [22].

Participant recruitment and follow-up

Consecutive women with pregnancies complicated by GDM, who delivered between September 7, 2020, and January 31, 2023, were approached by a researcher at the postnatal ward on the second day after delivery. These women received a study outline and were invited to participate in the study. All interested women were screened for eligibility. Written informed consent was obtained from all eligible women before the beginning of the study.

At enrollment, the clinical characteristics of the participants were collected from their medical records. These included age, weight, parity, family history of diabetes mellitus, severity of GDM, and gestational age at delivery. Participants were scheduled for serial assessments of weight and metabolic risk factors at 6 weeks and 6 months postpartum.

At 6 weeks postpartum, data on breastfeeding practices were collected from the participants upon their postpartum visit. Physical examinations, including measurements of weight, WC, and BP, were performed by a nurse who had been trained to ensure standard and accurate measurements. The body weight was measured without shoes and with light clothing using the Tanita Model WB-3000 digital scale (Tanita Corporation, Tokyo, Japan). WC was measured in the horizontal plane midway between the lowest ribs and the iliac crest with the participants in a standing position. After 10 min of rest, the BP was recorded to the closest 2 mmHg using the Nova-Presameter Desk mercury sphygmomanometer (Rudolf Riester GmbH, Jungingen, Germany) with the arm supported at heart level. Three separate BP readings were taken at 1-min intervals. The average of the last two readings was used for the analysis.

Venous blood samples were drawn to determine FPG, fasting lipid, and 2-h postprandial glucose (PG) levels. Fasting lipids included total cholesterol, TG, low-density lipoprotein cholesterol, and HDL-C levels. If participants were diagnosed with T2DM, MetS, or both, they were referred to an endocrinologist for further management.

At 6 months postpartum, the same data were collected for all participants. Blood samples were collected to determine FPG, fasting hemoglobin A1c (HbA1c), and fasting lipid levels.

Laboratory measurements

All blood samples for FPG, HbA1c, and lipid analyses were collected in the morning after overnight fasting for 12 h and sent to the hospital’s laboratory. A standard 75-g OGTT was performed to measure 2-h PG levels. Plasma glucose and lipid levels were measured using the automated analyzer Cobas c503 (Roche Diagnostics, Mannheim, Germany). HbA1c levels were measured using a Cobas c513 analyzer (Roche Diagnostics).

All devices were calibrated in-house daily and annually using external validation. Our laboratory has received National Glycohemoglobin Standardization Program certification for HbA1c assays and is traceable to the Diabetes Control and Complications Trial reference method. The HbA1c and blood chemistry analyses were approved by the Randox International Quality Assessment Scheme.

Stratification of participants, outcome assessment, and definitions

We stratified participants into three groups according to weight changes from 6 weeks to 6 months postpartum: weight loss (> 2 kg), weight stability (± 2 kg), and weight gain (> 2 kg) [17]. The outcome measures included changes in the prevalence rates of MetS and its components.

The presence of MetS was established using the joint interim statement of the International Diabetes Federation Task Force on Epidemiology and Prevention [5]. MetS was defined as the presence of three or more of the following five metabolic components: large WC (≥ 80 cm), elevated BP (systolic BP ≥ 130 mmHg, diastolic BP ≥ 85 mmHg, or both, or treatment with antihypertensive drugs), elevated FPG levels (≥ 100 mg/dL, or treatment with antidiabetic medications), high TG levels (≥ 150 mg/dL, or treatment with drugs for elevated TG levels), and low HDL-C levels (< 50 mg/dL, or drug treatment for reduced HDL-C levels). Those with FPG levels ≥ 126 mg/dL, 2-h PG levels ≥ 200 mg/dL, or HbA1c levels ≥ 6.5% were diagnosed as having T2DM [23].

Statistical analyses

All analyses were performed using IBM SPSS Statistics for Windows, Version 28.0 (IBM Corporation, Armonk, NY, USA). Categorical variables are presented as numbers and percentages and were compared using the chi-squared test. Continuous variables are described as means and standard deviations. Differences in the means of continuous variables between the three groups were analyzed using a one-way analysis of variance. When the overall analysis was significant, intergroup comparisons were made using the least significant difference method as a post-hoc test.

Changes in the means of variables over time (between 6 weeks and 6 months postpartum) within each group were analyzed using paired t-tests. The differences in changes between the three groups were analyzed using a one-way analysis of covariance, controlling for weight at 6 weeks postpartum and exclusive breastfeeding at 6 months postpartum, which were previously identified as factors affecting the prevalence of MetS or metabolic risk factors [17, 24]. Changes in the prevalence rates of MetS and its components over time within a group were calculated using McNemar’s test. The differences in changes between the three groups were compared using the chi-squared test. Statistical significance was defined as a two-sided p < 0.05.