Oh sorry, I will copy and paste!
usmoms.wordpress.com
OPK as HPT?
Yes; you can use a LH surge(ovulation predicting) test strip as a pregnancy test accuratly.
How?
Right after you ovulate your LH surge test strip will show up negitive unless you have not ovulated.
*If it shows up some what positive and than fades to a negitive after a few days you *may* not have ovulated (common after a miscarriage)
Than right before AF is due, if you are pregnant, it will slowly become positive again just like before you ovulated. Than eventually you will get a dark positive. If you suffer from a chemical pregnancy you will continue to get negitive results on the pregnancy test and positive results on the OPK until Af shows up.
*peeonastick.com
Want to know why it works?
When looked at under a microscope HCG(pregnancy detecting chemical) and LH (ovualtion detecting chemical) are almost identical. The HCG chemical is just a little more complex. The OPK is looking for a chemical that BOTH the LH and HCG have. There for right around AF is due you will almost no LH in your system. If you are pregnant, however, you will have HCG in your sysetm. This is why the test will turn positive if you are pregnant. You can not do it the other way around however and you should not rely on the test itself to confirm a pregnancy.
Why is it a better way to test for a pregnancy?
Unfortuently, you can not rely on the OPK to confirm pregnancy.If you could there would be no such thing as a pregnancy test and everyone would just use OPKs. I, however, like them because *for me* they build up to a positive BEFORE a HPT shows a faint line. I can easily see the OPKs getting darker and darker the closer i get to my expected AF.
Conclusion: They are good for earlier detection of pregnancy but you can not rely on them on their own.