I just went through this last year. My LO is 7mths now. I would hold off as long as possible to tell your parents (I was 16weeks when I did). I worked up until 3 days before I had my baby and took 4 weeks off and to be honest I was ready to go back after just a couple weeks. My parents understood that I would slow down leading up to my due date and after returning as well. The good thing for you is the weather will be getting cooler so you can take it easier if you need to.
I was scared to tell my families but they were so awesome and figured out short term care for the month and everyone came back. Good luck and congrats!