Do opks show a build up?

[MiissMuffet]

I've noticed with doing an opk everyday (at the same time each day) that every day they get a little bit darker until they reach positive. Does LH build up before theres a surge and do opks detect it throughout the cycle? Or is it just a coincidence?

 

[MrS. MaBrEy]

its different for everyone. some people get a progression and some people dont.