I would go based on ultrasound. My last cycle was in January found out I was pregnant in February based on the first day of my last period in January I should have been 5 weeks when I found out but was only really 3 weeks so my first due date was for October 24th and then when I had first ultrasound ended up changing it to November 7th so now today is October 24th I have had no signs of labor and I'll have two weeks left till November 7. Good luck and I know that every woman and her body is different
You last period isn't actually accurate: ( sorry we use the first 2 ultrasounds if you have early ultrasounds to determine baby size and when they will be ready to live outside of you. Fetal growth is the real marker for delivery. If your baby messured 11 weeks then he or she is somewhere close to 11 weeks. You will have a bump earlier with a 3rd baby.
I say go by 12 wks, the ultrasound will change later again when the baby grows more or might even match up.