Background: Characterization of dietary patterns represents a valid and meaningful measure of overall diet quality and nutrient intake. The study aims at evaluating the sociodemographic, lifestyle, environmental, and pregnancy-related determinants of maternal dietary patterns during pregnancy. Methods: The analysis was conducted on a group of pregnant women from the Polish Mother and Child Cohort (REPRO_PL). During the second trimester of pregnancy, 1306 women filled in a modified version of the validated food frequency questionnaire (FFQ). Dietary patterns were estimated using an exploratory factor analysis. Potential dietary determinants were assessed via a questionnaire or biomarker measurements (saliva cotinine level). Results: Two dietary patterns were identified. The Prudent dietary pattern was characterized by high consumption of fruit, vegetables, legumes, whole grains, poultry, and low-fat dairy products, while the Western dietary pattern included high intake of refined grains, processed meat, potatoes, and very low intake of whole grains. Clear sociodemographic, environmental, lifestyle, and pregnancy-related determinants of diet quality were established. Older age (β = 0.2; p < 0.01), higher educational level (β = 0.3; p < 0.01), and socioeconomic status (SES) (β = 0.2; p < 0.01), overweight/obesity before (β = 0.3; p < 0.01), as well as physical activity during pregnancy (β = 0.2; p = 0.01) were positive determinants of a healthier diet (increasing Prudent–Western difference in dietary pattern scores). On the other hand, parity (β = −0.2; p = 0.04) and living in small cities (β = −0.3; p = 0.02) were significantly related to a rather Western dietary pattern. Conclusions: The current study presents evidence on specific factors influencing dietary patterns. They need to be accounted for in educational programs and interventions focused on healthy diet recommendations during pregnancy.