2.2. Study selection

Studies were selected if they met the following inclusion criteria: (a) they contained original data from observational studies (cohort, case–control and cross‐sectional); (b) they were conducted on pregnant women of any age; (c) participants were free of pre‐pregnancy diabetes (type 1 or 2 diabetes), inflammatory and/or infectious diseases; and (d) OR and 95% CI were reported for the association of the higher quantile of hemoglobin and/or ferritin with the risk of GDM.

The Title and the Abstract of the studies was filtered first (Figure 1). Potentially relevant articles for the systematic review and meta‐analysis were selected for full exploration, which was carried out independently by two investigators (JCFC and NA).

Flowchart of study search and selection

From each article selected, we extracted information about its characteristics, such as author, publication year, geographical location, race/ethnicity, age of participants, study design, sample size, number of women with GDM, ascertainment of GDM criteria, trimester in which hemoglobin and/or ferritin was collected, the laboratory technique used to determine hemoglobin and/or ferritin, and its concentrations (Tables 1 and ​and2).2). If any of the data was missing, the authors were contacted for additional data. Furthermore, the quality of publications was evaluated using the STROBE Statement (von Elm et al. 2008).

Characteristics of studies included in meta‐analysis about hemoglobin levels and risk of GDM

ADA = American diabetes association; BMI = body mass index; CRP = C‐reactive protein; FHD = family history of diabetes; IADPSG = International Association of Diabetes and Pregnancy Study Groups; OGTT = oral glucose tolerance test; Q = quintiles; SD = standard deviation; T = tertiles; WHO = World Health Organization.

Characteristics of studies included in meta‐analysis about ferritin levels and risk of GDM

BMI = body mass index; CRP = C‐reactive protein; ELISA = Enzyme‐Linked immunoSorbent Assay; FHD = family history of diabetes; IRMA = ImmunoRadioMetric Assay; LGA = large for gestational age; OGTT = oral glucose tolerance test; Q = quintile; SD = standard deviation; WHO = World Health Organization.

To identify associations between hemoglobin and/or ferritin levels with risk of GDM, OR and 95% CI of the selected studies were used. These OR were log‐transformed (ln) to normalize its distribution. The 95% CI of each study was used to calculate the SE. Summary estimates were calculated by combining inverse‐variance using fixed‐effects model based on a common true effect from selected studies (Nikolakopoulou et al. 2014).