Well, the theory is that Bhcg is made of 2 different chain molecules - alpha and beta. The lutenising hormone that OPKs respond is made up of beta chains, and their molecular structure is similar to that of BHCG.

In early pg, the BHCG has more beta than alpha chains, so the OPK can be positive before a HPT, and later only the HPT will be positive.

I have heard this before, and never tested it...... this time, I did an OPK (felt funny and wondered whether I was PG) because I did not have any HPTs at home, and well, it was positive - that was last thursday.

On friday pm I did a HPT and came back as faintly positive - had to look in light, and so I did a bt at work which came back as equivocal with a BHCG of 16. Then on Sunday, the HPT was stronger, and BT was positive.

So, at least here in my case, it seemed to work because I was 2 days out from my period, and so definitely not ovulating for the OPK to be positive with....

Hope this helps, Good luck!!