Non-traditional risk factors for gestational diabetes mellitus : impact on prevalence and offspring birthweight

Gestational diabetes mellitus (GDM) is a common metabolic pregnancy disorder, affecting 1 in 7 pregnancies on a global level. In Finland, the prevalence of GDM has increased rapidly in the recent decades and was 21% in 2018. GDM is a major public health concern, with adverse short- and long-term health implications for the woman and her offspring. Traditional risk factors for GDM include advanced maternal age, overweight and obesity, a family history of diabetes and an ethnicity with a high prevalence of diabetes. There are, however, other risk factors, referred to as non-traditional, that have been shown to increase the risk for type 2 diabetes but have been studied less or with conflicting results with regard to GDM. The aim of this thesis was to evaluate the impact of several non-traditional maternal risk factors (height, body size at birth, smoking status and socioeconomic status) on the risk for GDM. The thesis is part of the register-based follow-up Vantaa Birth Cohort study 2009–2015. Data has been collected from the Finnish Medical Birth Register, the Finnish Social Insurance Institution, the Finnish Tax Administration, and Statistics Finland. In Study I, encompassing 4,111 Finnish primiparous women and their singleton offspring, maternal height was inversely associated with the development of GDM, after adjustments for age and educational attainment (p = 0.018 for linearity). Independently, both maternal height and GDM were positively associated with the birthweight of the offspring (calculated as Z-score according to sex and gestational age; p < 0.001 for both). However, the interaction between maternal height and GDM was significant and an increase in birthweight was noted only in women within extreme height categories, group I ≤ 158 cm (p = 0.011), group IV 168–172 cm (p = 0.010) and group V ≥ 173 cm (p < 0.001). In Study II, encompassing 1,548 Finnish primiparous women, there was a positive correlation between maternal body size at birth (assessed as body surface area [BSA]) and adult anthropometry. The association between maternal BSA at birth and GDM was inverse (p = 0.015 for linearity), after adjustments for age, educational attainment, pre-pregnancy body mass index (BMI) and smoking. In Study III, encompassing 4,111 Finnish primiparous women and their singleton offspring, a positive relationship between smoking during pregnancy and GDM was observed. The prevalence of GDM was highest in the group of smokers who continued smoking after the first trimester, compared with those who quit, and non-smokers (p = 0.004 for differences between groups). In women without GDM, birthweight was lowest in newborns of smokers who continued smoking after the first trimester (p = 0.004 for differences between groups, adjusted for age and pre-pregnancy BMI). In women with GDM, offspring birthweight was not related to maternal smoking. In Study IV, encompassing 5,962 Finnish primiparous women, there was an inverse association between increasing maternal income level and the development of GDM, after adjustments for age, cohabiting status, pre-pregnancy BMI and smoking (p < 0.001 for linearity). Educational attainment also showed an inverse relationship with the development of GDM. In conclusion, maternal current height, BSA at birth, and socioecomonic status (assessed as both income and educational attainment) were all inversely associated with the risk for development of GDM. Further, maternal smoking during pregnancy increased the risk for GDM. Birthweight was lowest in newborns of women without GDM who continued smoking after the first trimester. However, among newborns of women with GDM, birthweight was not related to maternal smoking. Recognizing specific maternal risk factors is important in lifestyle conseling and targeted prevention of GDM. Hence, findings on the non-traditional risk factors evaluated in this thesis indicate they are of importance both from a clinical and public health perspective. However, future studies are needed to confirm the associations, taking possible additional confounding factors into account, as well as to verify the mechanisms behind these associations.