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: UNASSIGNED : Our previous study has indicated that prenatal caffeine exposure ( PCE ) could induce intrauterine growth retardation ( IUGR ) of offspring .
### Output Text:
UNASSIGNED : Our previous study has indicated that prenatal <span class="chemical">caffeine</span> exposure ( PCE ) could induce intrauterine growth retardation ( IUGR ) of offspring . 