I'm a ftm, and I started showing at 11 weeks. Each person/pregnancy is different.
As far as the accuracy of the app, it has been spot on for me. I also knew when my last menses was to estimate my due date.
Your Dr will be able to clarify for you. They may do a sonogram prior to the 18-20 week anatomy scan. Good luck!!
I'm 13 weeks too! I feel huge, I am definitely showing.