BACKGROUND Although folic acid deficiency is known to be one of the factors in the development of spina bifida and other neural tube defects (NTD) the exact pathophysiology still remains unclear. Progesterone is an endogenous hormone which increases significantly during pregnancy. OBJECTIVE We aimed to study the possible negative effects of high dose progesterone on neural tube development in early chick embryos. In order to test our hypothesis, early chick embryos were exposed to physiological saline, normal and high doses of progesterone. METHODS 160 fertile, specific pathogen free white leghorn eggs (Gallus gallus), all at stage eight of development were divided into four equal groups. METHODS The first group was incubated without any operation. The second group was injected with physiological saline. The third and fourth groups were injected with two and twenty times more than physiologic doses of progesterone respectively. After 48 hours of incubation, all embryos were analyzed for the presence of NTDs under light microscopy. METHODS None. RESULTS At 48 hours of incubation, 84% (135/160) of the embryos passed characteristics of Stage 12 development and were included to the study. None of the eggs in the first three groups showed NTDs, whereas 81.8% (27/33) of the eggs in the fourth group showed NTDs. CONCLUSIONS Our study showed that progesterone at levels twenty times more than its physiologic level might cause NTDs. Further studies are needed to explain the mechanisms of this teratogenic effect.