cajunbabe
Mama to Miss Lillian!
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The OPKs tell you the day you surge. Which means ovulation has either already happened if you catch the surge downward, or it's about to happen in 12-24 hours on the upward surge. The CBFM tells you when your levels are rising so you actually get 4 days of warning. I think the problem with the OPKs is you don't get enough of that warning. The CBFM measures two different hormones to give a more accurate picture.