In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: CONCLUSIONS : Reassuringly , penicillins , erythromycins , and cephalosporins , although used commonly by pregnant women , were not associated with many birth defects .
### Output Text:
CONCLUSIONS : Reassuringly , <span class="chemical">penicillins</span> , <span class="chemical">erythromycins</span> , and <span class="chemical">cephalosporins</span> , although used commonly by pregnant women , were not associated with many birth defects . 