Background: In the last decades, plant-based diets have gained popularity. Pregnancy is not a contraindication to follow a meat-free diet. This study aimed to compare maternal and neonatal outcomes between women who followed a plant-based diet with those on an omnivore diet. Our second purpose was to investigate the association between physical activity level in combination with diet type and the occurrence of GDM and gestational hypertension. Methods: A questionnaire was distributed electronically via social media. The survey was conducted on a population of Polish women. Results: The final research group included 1015 women. The results showed that a maternal plant-based diet 6 months before pregnancy and during pregnancy does not change the incidence of GDM, anemia, and gestational hypertension. Moreover, no association was found between a diet type before conception and a delivery method or newborn birth weight. Among women who followed an omnivore diet, the risk of GDM was lower in a group with adequate physical activity during 6 months before conception (p = 0.0166). However, the combination of a plant-based diet with adequate activity during the preconception period did not influence GDM incidence. Conclusions: Our study indicates that a plant-based diet during the preconception period is not worse than an omnivore diet.