A mother took thalidomide for morning sickness; her baby has flipper-like limbs. Thalidomide, taken during weeks 4–8 when limbs form, disrupts limb bud outgrowth — phocomelia (seal limbs), the tragedy that created modern drug-safety testing. The critical principle: teratogens harm organs only while they're forming. The same drug later in pregnancy might be harmless to limbs but the brain, still developing, stays vulnerable longer — timing decides the defect.