Maternal diet during pregnancy, gestational weight gain and preconceptional Body Mass Index (BMI) influence fetal development and the future health of both mother and child. The aim of this longitudinal study is to evaluate the effect of prepregnancy BMI and diet quality throughout pregnancy on pregnancy outcome. To analyze diet quality a 7-day food record of 141 subjects, categorized according to prepregnancy BMI, was collected during each trimester of pregnancy. Diet quality was scored by the Healthy Eating Index (HEI). Pregnancy outcome was prospectively registered. The results showed that only 24% of the pregnant women had a healthy diet as indicated by a HEI score above 80. The mean HEI scores did not change during pregnancy and decreased with increasing prepregnancy BMI. The prevalence of hypertensive disorders and labor induction was significantly higher among obese women compared to normal weight women. Seventy-two percent and 47% of the overweight and obese women had an excessive gestational weight gain compared to 6% and 24% of underweight and normal weight women (p = 0.001). An inverse relationship was observed between prepregnancy BMI and the initiation of exclusive breastfeeding. Weak, non significant associations between HEI and pregnancy outcome were detected. The concluding observation is that diet quality decreased with increasing BMI, whereas the prevalence of pregnancy complications and excessive weight gain increased with increasing BMI. The contribution of an unhealthy diet to adverse pregnancy outcome, however, appeared to be limited in the group of women studied.