Why is early alcohol exposure (before many people know they are pregnant) of particular concern for fetal development?

The embryo in early pregnancy has heightened alcohol-metabolizing enzymes that rapidly clear ethanol and therefore early exposure is protective, making later pregnancy exposures more concerning only after organ systems mature.

Early exposure only affects maternal tissues and the embryo is completely shielded until the third trimester, so early drinking is essentially harmless to fetal development.

Early embryonic development includes critical periods of organogenesis during which the embryo has limited metabolic capacity, so toxic exposures can produce permanent structural and functional defects even before pregnancy is detected.

Early alcohol exposure stimulates fetal growth factors that always lead to above-average birth weights and no neurological harm, so the concern is largely sociocultural rather than biological.