The “all-or-none” concept is a classic teratology framework used for the very earliest stage of pregnancy—around fertilization and implantation—when embryonic cells have not yet differentiated into organs. The idea is simple: if an exposure causes significant damage at this stage, the embryo typically cannot continue developing and the pregnancy does not progress (“all”). If the damage is minimal or repairable, development continues normally without a specific structural defect (“none”). Clinicians often use this concept to reduce panic after an accidental one-time exposure before someone knew they were pregnant. However, it has limits. Timing matters: once organ formation begins, exposures can have more specific and lasting effects. Dose and duration matter as well: a single small exposure is not assessed the same way as repeated high exposure. Therefore, “all-or-none” is not permission to ignore risks; it is a way to contextualize early, low-level accidental exposures and guide a calm, clinician-led evaluation.