Why is this app always like a week ahead?

Ow ok I get it now thank you

So you're 13 weeks along, but it's the 14th week

Because you are 13 weeks pregnant but in your 14th week of pregnancy.

Because it's the 14th week. To put it simply the first week of your pregnancy(your period technically) you're not one week along until the end of the week, but it's still the first week. Make sense?