The influence of prepregnancy body mass index (BMI) and dietary patterns on pregnancy outcomes remains unclear. This study examines the sociodemographic factors affecting the prepregnancy BMI and dietary health, as well as their impact on maternal and neonatal complications. A total of 1064 women were enrolled at the Jinan Maternal and Child Health Hospital (Shandong, China) from January 2021 to December 2023. The China pregnancy healthy diet index (CHDI-P) was used to assess dietary health. Regression analyses were conducted to evaluate the relationship between sociodemographic characteristics, the BMI, CHDI-P scores, and adverse pregnancy outcomes. Higher education and moderate income were protective factors for maintaining a normal BMI, while older maternal age was linked to dietary patterns. Prepregnancy obesity significantly increased the risk of gestational diabetes and hypertension, while overweight and obesity were associated with a lower risk of small-for-gestational-age (SGA) births. Additionally, suboptimal dietary patterns were linked to a higher risk of large-for-gestational-age (LGA) infants and macrosomia. Prepregnancy overweight, obesity, and unhealthy dietary patterns contribute to adverse pregnancy outcomes, including gestational diabetes, hypertension, LGA, and macrosomia. These findings highlight the importance of weight management and nutritional guidance before and during pregnancy, particularly for women with lower educational attainment.