Abstract Objectives Adequate maternal nutrition is of paramount importance in pregnancy, particularly in the first trimester when fetal development is crucial. It has been reported that heavier women are most likely to fear weight gain associated with pregnancy. Few studies have, however, investigated associations between prepregnancy body mass index (BMI) and nutrient intakes in the first trimester of gestation using detailed, prospective methodologies. The aim of this study was to investigate possible associations between prepregnancy BMI and nutrient intakes in the first trimester of gestation. Design Seventy-two Caucasian, primiparous nonsmokers of mean age 33.1 years (SD 4.6) were recruited from three London teaching hospitals and they completed a background information questionnaire and a 4- to 7-day weighed inventory food diary during the first trimester of pregnancy. Prepregnancy anthropometric data were extracted from General Practitioner records. Results Prepregnancy BMI was inversely associated with dietary energy (P = 0.04), Southgate and Englyst fibre (P < 0.01), and iron and folate (P < 0.01). After excluding under-reporters [individual energy intake:basal metabolic rate (estimated) ratio < 1.2], prepregnancy BMI was inversely associated with folate intake (P =0.04). Dietary intakes of Englyst fibre (P = 0.03) were statistically significantly lower than average dietary recommendations in this group. Conclusions This study identified that women with a high prepregnancy BMI are more likely to under-report nutrient intakes. The finding that folate intake was significantly lower in heavier women accurately reporting dietary intake is of particular concern.