I use OPKs under the guidance of my RE (specialist). She has us DTD once a day for three days following our first + opk. That's it. Before that we try to DTD about every 48 hours just in case/for kicks/keep the swimmers fresh, but it's not "required"!
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OPK results seem to vary quite a lot from woman to woman. The first month using them, you might have to experiment a little to see what works for you. For example, for me it's very easy to tell a + from a -, because my + is clearly darker than the control line. But I know a lot of women who won't get the + but will catch a near positive only, in which case they will count their + differently from how I count mine. Also, I almost always get a fade-in pattern (the line gets a little darker every day) so I have a clue as to when I am going to surge and I don't test more than once a day - I don't need to.
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I know a lot of brands say test in the early afternoon or night, but the first brand I used said to use FMU and that's what I've always done - works fine for me. If you're not sure, you could try testing twice a day around your usual o date. If you don't know it, I'd say estimate by counting 14 days back from your average cycle length and start about 2 days earlier than that with testing twice a day. I think starting at day 7 is a good idea for your first cycle using OPKs, but I wouldn't test twice daily until about cd10-12 unless you know you tend to ovulate earlier in your cycle.
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FWIW, I use the cheapies sold by Amazon and they work great for me.Â
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Hope that helps! GL!