Cancelling a day trip with girlfriends on Saturday?

Audrey • 31 years old. Mom of one little boy 👶🏼 with another bundle on the way! Happily married since ‘16 💏
Hey ladies I'm at 9 weeks and the past two or three weeks I've been absolutely exhausted. I'm just able to go to work and drive home and I go to bed at 7:30 or 8 every night. 
I have a day trip planned this weekend but now I'm thinking it would be better to stay home and rest. I feel bad because my friends had to change the date for me before and now I want to cancel but I feel like I should do what's best for the baby. Any ideas for a good excuse to bow out? Thoughts? 
Thanks in advance