Week Timing Question

Liahona

I have 2 <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">pregnancy apps</a> on my phone, this one and Baby Center.

This app says that today I am 5 weeks exactly. Baby Center says that I am 4wk 7d and tomorrow will be week 5.

So, which is correct?