Maternal thyroid function, prepregnancy obesity and gestational weight gain—The Generation R Study: A prospective cohort study

Summary Objective Maternal prepregnancy obesity and excessive gestational weight gain are associated with pregnancy complications. Thyroid function is related to differences in body mass index (BMI) in adult populations. We examined the associations of maternal thyroid function in early pregnancy with maternal BMI and weight gain during pregnancy. Design and methods In a population‐based prospective cohort study among 5726 mothers, we measured maternal TSH and FT4 levels at 13.5 weeks of gestation (95% range: 9.7‐17.6 weeks). Maternal weight was assessed before pregnancy and in each trimester. Results Higher maternal TSH levels were associated with higher prepregnancy BMI (difference: 0.18 kg/m2 [95% CI: 0.01, 0.36] per SD increase in maternal TSH level) and higher total gestational weight gain (difference: 0.02 kg/wk [95% CI: 0.01, 0.03] per SD increase in maternal TSH level). Higher maternal FT4 levels were associated with lower prepregnancy BMI (difference: −0.44 kg/m2 [95% CI: −0.63, −0.26] per SD increase in maternal FT4 level) and lower total gestational weight gain (difference: −0.01 kg/wk [95% CI: −0.02, −0.01] per SD increase in maternal FT4 level). The associations of maternal thyroid function with weight gain in early pregnancy were stronger than those with weight gain in mid and late‐pregnancy. Maternal hypothyroidism was associated with higher prepregnancy BMI and early pregnancy weight gain, whereas opposite effects were observed for maternal hyperthyroidism (P<.05). Conclusions Higher maternal TSH level and lower FT4 level in early pregnancy are associated with higher prepregnancy BMI and higher gestational weight gain. Further studies are needed to explore maternal and foetal consequences.