Developmental adaptations due to early nutritional exposures may have permanent health consequences. Studies of diet and fetal size have mainly focused on individual nutrients despite evidence that the pattern of food consumption may be of significance. Hence, we evaluated the associations of dietary habits in early pregnancy (gestational age < 18 weeks) with fetal size, uteroplacental vascular resistance, placental weight and birth weight in a prospective observational study of 3207 Caucasian pregnant mothers in Rotterdam, the Netherlands. Participants completed a semiquantitative FFQ during early pregnancy. Logistic regression analysis was used to predict the occurrence of intra-uterine growth retardation at birth as a function of food intake. The derived solution was considered as the dietary pattern. As it was characterised by higher intakes of fruit, vegetables, vegetable oil, fish, pasta and rice, and lower intakes of meat, potatoes and fatty sauces, it was labelled the 'Mediterranean' diet. The degree of adherence to the diet was positively associated with plasma folate and serum vitamin B12 concentrations and showed an inverse relationship with homocysteine and high-sensitivity C-reactive protein plasma concentrations (P <0·05). Important fetal size and placental parameters were associated with the degree of adherence to the diet, revealing a 72 g lower birth weight (95% CI -110·8, -33·3) and a 15 g lower placental weight (95% CI -29·8, -0·2) for women with low adherence to the diet. To conclude, low adherence to a Mediterranean diet in early pregnancy seems associated with decreased intra-uterine size with a lower placental and a lower birth weight.