When comparing baby's size to objects what weeks do you use?

Erin

The glow app says I'm 3 weeks and 5 days, which I'm guessing is around the first day of my last period. And when I take a pregnancy or blood test I'm 2 weeks pregnant.

I see cute things online and on Pinterst about comparing the baby's size to poppy seeds, oranges, etc but I'm not sure what which reading I'm supposed to use?