The course of human development during intrauterine life, particularly during embryonic development is extremely complex. A huge number of biochemical, physical and organizational processes must be precisely coordinated to assure normal development. Many of these events are very critical due to their irreversible nature and their short duration besides, they may be sensitive to a lot of teratogens .These stages, which are called" critical developmental stages" have attracted special attention of the researchers to the effect of different teratogenic agents during the critical stages of conception. This paper reviews some molecular events of various stages in human development during embryonic as well as fetal development besides, critical periods, involved molecules in some body organs-including glycoconjugates, morphogens, etc. have also been explained. It is emphasized that identifying these critical processes would help our colleagues to offer the requisite points to pregnant women so that they would avoid undue taking of specific drugs and chemicals. This will hopefully prevent many of inborn teratogeneses.