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: During a ten - year period , 348 women were studied for a total of 5 , 877 patient months in four separate studies relating oral contraceptives to changes in hematologic parameters .
### Output Text:
During a ten - year period , 348 women were studied for a total of 5 , 877 patient months in four separate studies relating <span class="chemical">oral contraceptives</span> to changes in hematologic parameters . 