meaning - From the third week ( when does it actually begin)

This describes a certain natural process that takes 15 weeks

It describes what a certain component looks like at some point in time during a naturallly occurring process

What I don't get is if it means it appears this way starting in the third week or after ?

Here's the full sentence

"The malleus first appears as a cartilage model from the third week"

Thanks again for you support guys