Opk's are not for everyone. They can be very frustrating but there is nothing like the feeling of knowing when you are ovulating.
OPK's detect LH a hormone that is released just before ovulation. You want to BD on the day you get a positive as well as a few days before and a few days after a positive to maximize your prospects.
When you need to start using opk's is dependent on your cycle. Usually they say to start around cd10 and you use them daily until you get a positive. Each test is different but you either hold it in your stream or urinate in a cup and put the test in it. Then comes the waiting. Some test are digital and give you a positive or negative sign. Most give you a control line and then another line appears dependent upon the amount of LH in your body. As you get closer to ovulation the second line will get stronger (sometimes) until it is the same color as the control line. Then after you ovulate it will disappear (usually).
I use opk's and to my knowledge I do not have any fertility issues. I just want to maximize my chances of getting pregnant. Really they aren't that difficult to do. I do it ever afternoon when I get home from work. You only need to limit your fluids for about 2 hours before. If you get them from the internet they are really cheap.
I am the type of person who really takes charge of whatever I am into, so I decided to use them, but for thousands of years women have been getting prego without them.
Hope that helps!
