I need some time off work.

Hi ladies. I'm 16 nearly 17 weeks pregnant now I'm coping with being pregnant fine but I need some time off work I'm exhausted, stressed work isn't helping at all and I'm just so down at the moment I really just need some time away to focus on me and my baby. 
What can I say to my doctor for him to supply me with a sick note? 
Any advice is greatly appreciated xx