Can I ask for a c-section?

Yes. You can ask, but if your exam shows your pelvis to be adequate for vaginal delivery, your provider should kindly refuse your request. The risk to both you and baby are increased in c-section, and they should be reserved for medical reasons only.
Discuss with yor OB. Acog says that u can have an elective csection to be scheduled at 39-40 weeks as long as you have been properly told of the risks and benefits of that, I frequently do that for my patiets.