How many weeks do you start to request to wfh?

Hi mummies, at how many weeks do you start to request to wfh? I’m 29 weeks now and I feel so heavy and breathless all the time. My company require us to go back 4 days a week and the transport to and fro work is taking a toll esp when baby is pressing very low. Did anyone manage to get gynae to write a letter just to request to wfh?