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: When they do occur they are often unrecognized because of denial but when significant may pose a risk to both the mother and her fetus .
### Output Text:
When they do occur they are often unrecognized because of denial but when significant may pose a risk to both the mother and her fetus . 