Crib mattresses are described as firm on nearly every package, and the word carries a specific technical meaning. It refers to a measurable property with a defined test rather than a marketing claim.
Firmness is measured, not asserted
Infant mattress standards define a test in which a weighted hemispherical form is applied to the surface and the resulting indentation is measured.
The measurement produces a number that either falls within the permitted range or does not. It removes the judgment that a hand press involves.
Testing is performed by accredited laboratories, and manufacturers of products in the regulated category must certify compliance on that basis.
The physiology behind the requirement
Infants have limited head control and cannot reliably reposition themselves. A surface that conforms around the face can restrict airflow.
Firm surfaces resist that conforming, and the standard exists to define how much resistance is enough rather than leaving it to description.
This is also why the standards address the fit of the mattress within the crib frame, since gaps create a separate hazard from the surface itself.
Adult comfort and infant safety point opposite ways
Adult mattress design has moved toward contouring surfaces, which is the property infant standards restrict. Consumer intuition about comfort works against the requirement here.
Products marketed as more comfortable, softer or contoured for infants may not be intended as sleep surfaces at all, and their labeling should be read closely.
Add-on items such as toppers, wedges and covers change the tested surface, and manufacturers generally state that they are not to be used in a crib.
Regulation covers a defined product category
Federal rules for durable infant and toddler products bring specific categories under mandatory standards, including cribs and infant mattresses.
Products outside those categories can be sold without meeting the same requirements, which is why category labels on packaging carry real information.
Sleep product regulation in the United States has been revised over the past several years, so older items may predate the current requirements.
Where sleep guidance comes from
Safe infant sleep recommendations are published by pediatric professional bodies and federal agencies, and they cover surface, position and environment together.
A compliant mattress is one element within that guidance rather than a substitute for it, and the published recommendations are the authoritative statement.
For questions about a particular infant's sleep, a pediatrician is the right person to ask rather than a product description or a general article.