Objective The present study examined nutritional status and dietary intake of pregnant women in Japan in relation to pre-pregnancy body mass index (BMI).Methods Participants included 141 Japanese women with singleton pregnancies, from the outpatient department of the S hospital, Gunma prefecture, Japan. Two-day food records, dietary assessment questionnaires, and clinical records were obtained at 20 weeks gestation. Nine patients were excluded from the study due to morning sickness. The remaining 132 participants were divided into 3 groups according to pre-pregnancy BMI: underweight, normal weight, and overweight. Nutritional status and dietary intake were analyzed in relation to BMI using the chi-square test, Fisher's exact test, Kruskal-Wallis test, one-way analysis of variance, and analysis of covariance with adjustment for age, employment status, and total energy intake.Results Women who were underweight before pregnancy were more frequently working full-time than normal weight and overweight women. Underweight women were also more frequently anemic (P=0.038, underweight 39.3%, normal weight 24.7%, overweight 0%) and had lower mean hemoglobin (Hb) (P=0.021, underweight 11.3 g/dL, normal weight 11.6 g/dL, overweight 12.1 g/dL) and hematocrit (Hct) levels (P=0.025, underweight 33.7%, normal weight 34.3%, overweight 36.0%). Their dietary intake of protein, iron, magnesium, and folic acid was lower than that of normal weight and overweight women. Their meals tended to include fewer meat, fish, egg, and soybean dishes (underweight, mean of 4.7 servings per day; normal weight, 6.1 servings; overweight, 6.1 servings).Conclusion Pregnant women who were underweight before pregnancy had increased risk of anemia as well as reduced Hb and Hct levels. They had lower dietary intake of protein, iron and folic acid compared to women in the other BMI categories. Anemia and these nutrient deficiencies are known risk factors for low birth weight. Our findings suggest the importance of providing underweight pregnant women with support to improve dietary intake during their pregnancy, especially to increase intake of protein and iron through consumption of fish and meat dishes.