The first trimester is a sensitive window because early development accelerates. It helps to separate “clearly contraindicated without medical guidance” from “best avoided when unnecessary.” Clinically, avoid self-medicating; do not use alcohol or tobacco; manage food-borne risks (raw/undercooked foods); and reduce infection exposure. For cosmetics, the practical rule is to avoid high-risk/high-dose topicals: stop retinoid-based acne/anti-aging products, postpone strong chemical peels, and avoid hydroquinone-type lighteners unless your clinician advises otherwise. Heavily fragranced or harsh exfoliating routines can also worsen irritation and barrier disruption, creating avoidable problems. The “all-or-none” concept is often used to contextualize very early accidental exposures: extremely early on, exposures tend to result in no effect or pregnancy loss; once organ formation progresses, effects can be more specific. This is not a license for risk, but a way to reduce panic after a one-time mistake. The best first-trimester approach is a simplified routine, robust sun protection, and clinician-aligned decisions for any potent active ingredient.