Independent variables
The main independent variables were the mode of conception (spontaneous / IVF/ICSI) in the observational study, and infertility with reproductive surgery in the case-control study (yes / no).
Independent co-variables were maternal age (continuous), maternal age at delivery >35 years, pre-pregnancy body mass index (BMI) (continuous), BMI ≥ 30 kg/m2 (yes / no), BMI < 18.5 kg/m2 (yes / no), gestational weight gain (GWG) z-score (continuous), GWG z-score > -2 (yes / no), GWG z-score > 2 (yes / no), pre-pregnancy hypertension (yes / no), pre-pregnancy diabetes (yes / no), previous conization (yes / no), uterine anomalies (yes / no), chronic kidney disease (yes / no), other chronic diseases (heart, thyroid and/or mental disease) (yes / no), previous preterm delivery (yes / no), previous miscarriage (yes / no), smoking (yes / no), preeclampsia/eclampsia (yes / no), gestational diabetes (yes / no), small-for-gestational age neonate (birthweight < 10th percentile) (yes / no), nulliparous (yes / no).
BMI was defined as the woman’s body mass divided by the square of body stature (kg/m2). The pre-gravid BMI was categorized according to the Institute of Medicine criteria as underweight (< 18.5 kg/m2), normal weight (18.5–24.9 kg/m2), and overweight (15.0-29.9 9 kg/m2), and obese (≥ 30 kg/m2)12. In Slovenia, maternal weight is recorded at the first appointment in pregnancy and all appointments after. The mean gestational week at the first appointment in pregnancy was nine weeks. For the calculation of the GWG z-score, we used the same method as described before13. Preeclampsia was defined as the presence of hypertension (blood pressure ≥ 140 and/or 90 mmHg) and any other end-organ dysfunction after the 20th week of gestation14. National guidelines for screening and diagnosing GDM changed during the study period. Until 2012, GDM was diagnosed in Slovenia using the two-step approach: screening with 50 g oral glucose load and, when indicated, performing a 100 g oral glucose tolerance test (OGTT) using the Carpenter and Coustan criteria15. Since 2012, we diagnose GDM using the one-step approach by a 75-g OGTT according to the 2010 International Association of Diabetes and Pregnancy Study Groups Consensus16. SGA was a neonate with birthweight below the 10th percentile for gestational age and gender-based on a Slovenian Birth Weight Standards17. A nulliparous woman was defined as a pregnant woman who had no previous viable pregnancies.