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Context: Obesity and too much weight gain during gestation have a negative effect on obstetric and neonatal outcomes., Objective: To determine the relationship between thyroid hormone parameters, body mass index (BMI) and weight gain during gestation., Design: Prospective follow-up study of thyroid parameters and gestational weight gain., Setting: Healthy pregnant women, included at first antenatal consultation., Patients: Thyroid function (TSH, FT4 and TPO-Ab) was assessed at 12, 24 and 36 weeks' gestation in 1035 Dutch Caucasian women who delivered at ≥37 weeks. BMI (WHO criteria) was assessed at eight weeks, and weight gain throughout gestation was also assessed using the US Institute of Medicine (IOM) criteria., Primary Outcome Measure: a possible relationship between maternal thyroid parameters and BMI at the first trimester., Secondary Outcome Measure: the relationship between thyroid parameters and weight gain throughout gestation., Results: At 12 weeks' gestation, BMI correlated with FT4 (r = -0·14, P < 0·001), but not with TSH (r = 0·04, P = 0·89). 415 (40%) of the women met the IOM criteria for appropriate weight gain, 326 (32%) showed less weight gain and 294 (28%) gained too much weight. At all trimesters, the latter group of women showed higher median TSH and lower median FT4 compared with those with normal weight gain. FT4 at 24 weeks' gestation (OR: 0·84, 95% CI: 0·77-0·91), younger age (OR: 0·97, 95% CI: 0·95-0·99) and primiparity (OR: 0·51, 95% CI: 0·38-0·68) were independently related to too much weight gain., Conclusions: Maternal thyroid parameters are related to both prepregnancy BMI and weight gain throughout gestation., (© 2013 John Wiley & Sons Ltd.)