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CM means you are fertile, then when you O you should notice a sudden decrease in CM. For avoiding I would stop when I saw fertile CM then continue again 3 days after the last day of fertile CM. I don't think I'd bother with OPKs but that is because I feel CM was my best predictor.

I think OPKs are sketchy for preventing pregnancy. Firstly, you should get a + 12-48 hours before you ovulate, but sperm can live for up to 5 days so say you had sex the day before a positive OPK... you wouldn't be safe.

Also, you can get a + OPK but then not ovulate at that time, so unless you take them for the whole cycle you might think you've ovulated only to ovulate later...

I think CM is much more reliable but still not fool proof.

Temping would definitely be a better way to avoid pregnancy... OPKs and CM are both pretty unreliable ways of pinpointing O. If your cycle is very regular than you might be able to use these methods to prevent decently.

Like a PP said, sperm can live in your body for days, so if you had sex the day before a positive OPK if might be too late to prevent pregnancy. The only time you could be fairly certain that sex won't lead to conception (although still not 100%) is 4-5 days after your positive OPK, since once you have O'd you can't get pregnant... The trouble is you can't be completely sure of when you O'd without charting.

@Bubbles: If you google resources for "Natural Family Planning" (NFP, the method used by some practicing Catholics) there is a ton of stuff out there.

OPKs are also expensive. Fertility Friend is a great app.