One of the more pervasive tenets of teratology is the “all-or-none” phenomenon, which refers to the concept that embryonic exposure that occurs before organogenesis results in either no adverse embryonic outcome or in embryonic death. This concept has been used extensively in genetic counseling of pregnant women who have inadvertently undergone an exposure in the very early stages of pregnancy, frequently before the pregnancy has been recognized. Herein, we review the data that supports the all-or-none concept and the exceptions to this general rule. In the absence of further human evidence to the contrary, and given the many women exposed to medications or environmental agents before learning of their pregnancies, it would be prudent to continue to counsel pregnant women using the all-or-none hypothesis to avoid needless interruption of pregnancy out of unfounded fear of an adverse pregnancy outcome. Birth Defects Research (Part A) 94:664–669, 2012. © 2012 Wiley Periodicals, Inc.