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: Impact of alcohol exposure after pregnancy recognition on ultrasonographic fetal growth measures .
### Output Text:
Impact of <span class="chemical">alcohol</span> exposure after pregnancy recognition on ultrasonographic fetal growth measures . 