Continuous exposure of pregnant females to thiamethoxam (THM) during the organogenesis period causes reproductive toxicity through oxidative stress mechanisms, changes in fetal size and weight, (visceral and skeletal) malformations, histological changes in tissues, and oxidative stress in dams. Aim: This study aimed to investigate the toxicological effects of 1/10 and 1/20 of THM LD50 and the protective effect of vitamin C (vit C) on pregnant females and fetuses. Methods: Forty-eight female rats were divided into six equal groups and were orally treated daily with THM, vit C, and their combinations during the organogenesis period [6th–12th gestation day (GD)]. All pregnant female rats were slaughtered on GD 20. Blood samples, tissues (brain, liver, kidney, and lung), fetuses, and the uterus were examined. Results: Females orally treated with 1/10 and 1/20 of THM LD50 showed the highest weight reduction. Fetal length and weight were significantly decreased in groups treated orally with both doses of THM LD50. A significant increase in the number of resorption sites was recorded in 1/10 of the THM LD50 group compared with the groups treated orally with both doses of THM LD50 combined with vit C. Exposure of dams to high and low doses of THM LD50 resulted in various malformations in the fetuses’ skeletal, palate, cerebral ventricle, cerebrum, liver, kidney, bladder, and ureter. A significant increase in the percentage of changes in the skull, ribs, sternum, and vertebral column was observed in both THM doses. The degree of malondialdehyde was significantly increased in groups treated orally with both doses of THM LD50, while the total antioxidant level was significantly decreased. Conclusion: The results showed the potential of vit C in decreasing teratogenic effects on dams and fetuses exposed to THM, as it reduced the negative effects on different organs of the treated rats.