This paper uses data from antenatal and perinatal studies from Pelotas’ Birth Cohort of 2015 (C2015), which was conducted in the city of Pelotas in southern Brazil. The study invited all women living in the urban area of Pelotas who gave birth, including stillbirths, in the five maternity hospitals of the city, between January 1 and December 31, 2015.
Mothers were interviewed at the maternity hospital a few hours after delivery and answered a standardized questionnaire. They answered questions about antenatal period: demographic, socioeconomic, biological and behavioural issues, along with characteristics of pregnancy, delivery and medication use, including folic acid, iron salts and other vitamins and minerals. Furthermore, 75% of mothers who participated in the perinatal study were followed since the antenatal period by this cohort. More details of the study can be found in the cohort profile paper (16).
The analyses in this article were performed with all mothers from a perinatal follow-up who were eligible for this study (N = 2463) (Fig. 1). Anaemic pregnant women before 24th week (with record of at least one hemoglobin test < 11g / dL in the pregnant woman's antenatal card), were not included in the analyses.
For the outcome “gestational diabetes mellitus” self-reports were considered. The mothers answered the following question: “Did you have diabetes during pregnancy?”, followed by other question: “Did you have diabetes before pregnancy?”. Pregnant women who already had diabetes before becoming pregnant (n = 44) were excluded. Self-reported information about mother's knowledge of GDM, in the immediate postpartum, was validated through a study previously carried out in Pelotas' maternity, with high specificity (99.0%, 95%CI: 98.1; 99.6) and good sensitivity 73.0% (95%CI: 55.9; 86.2)(17).
The variables used in the analysis as possible confounding factors were: maternal age (collected in complete years and categorized as ≤ 19, 20–29, 30–46); ethnicity (self-reported by mothers as white, black or other); parity (total number of deliveries, including stillbirths and current pregnancy; later categorized as 1, 2, 3 or 4 or more); mother’s schooling (number of years of study, later categorized into four groups: 0–4, 5–8, 9–11 and 12 or more years) and family income expressed in local currency and converted into a multiple of minimum wage at the time of the perinatal interview (categorized as ≤ 1, 1.1 to 3.0, 3.1 to 6.0, 6.1 to 10 and > 10). A ‘minimum wage’ is a measure of the legal minimum monthly salary for formal employees in Brazil.
Family history of diabetes mellitus was reported by the mother at the 24-month follow-up, pre-pregnancy body mass index (BMI) was calculated by dividing pre-pregnancy weight by the square of maternal height, and categorised according to WHO criteria(18): underweight (< 18.5 kg/m2); normal (18.5–24.9 kg/m2); overweight (25.0–29.9 kg/m2); and obese(≥ 30 kg/m2) and smoking in pregnancy was considered “yes” when the mother reported smoking at least one cigarette a day, for at least 30 days.
Information regarding supplement use was taken from the following questions: " Have you used or are you using any vitamin, calcium, folic acid or iron salts since you became pregnant?”. If yes, the drug names were then questioned and for each drug reported a question about the trimester of use was asked: “In which trimester of pregnancy did you use this medicine?” (1st trimester / 2nd trimester, 3rd trimester).
From these questions, it was possible to generate the main exposure used in our analyses: “use of prophylactic iron in the first and / or second trimester of pregnancy”, which has been formulated using all iron compounds alone or in combination with other active substances, provided that iron was the main compound present. All analyses were performed in Stata software 15.0. Sample description was performed according to exposure and outcome (GDM) using the chi-square test. The association of iron supplements use with development of GDM was evaluated using logistic regression
The regression followed a previously established hierarchical conceptual model, which comprises three levels. The distal level included the sociodemographic variables and family history of diabetes; the second level included pre-gestational BMI, parity and smoke; and the proximal level included the use of iron salts. Variables with p < 0.20 were kept in the model to control confounding factors. For all statistical analyses, the significance threshold was set at p < 0.05.
The Federal University of Pelotas, School of Physical Education Ethics Committee, approved the study protocol (522.064). All mothers signed an informed consent form before being interviewed.