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: Perinatal risk factors of infants with hearing loss were compared with those of two control groups matched for gestation and birth weight and for perinatal complications .
### Output Text:
Perinatal risk factors of infants with hearing loss were compared with those of two control groups matched for gestation and birth weight and for perinatal complications . 