What does a pregnancy warning of 'D' indicate?

A pregnancy warning of 'D' indicates that there is evidence of human risk during pregnancy, but the potential benefits of the drug may still outweigh the risks in certain situations. This classification is crucial for healthcare providers when considering treatment options for pregnant patients. It signifies that there have been studies or data showing that the drug may have adverse effects on fetal development, thus alerting both providers and patients to exercise caution.

In the context of drug classifications, 'D' is more serious than 'B', which suggests no human risk but potential animal studies indicating some risk, confirming that the presence of risk in human subjects is what warrants the 'D' designation. Likewise, while 'A' indicates no known risk and option for 'not to be used in pregnant women' speaks to a complete contraindication, 'D' does not carry such a severe exclusion but rather serves as a warning requiring a risk-benefit analysis before administration.