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: Antibacterial medication use during pregnancy and risk of birth defects : National Birth Defects Prevention Study .
### Output Text:
Antibacterial medication use during pregnancy and risk of birth defects : National Birth Defects Prevention Study . 