Sasha... Sometimes you *can* miss it, if you test in the morning and the surge is in the afternoon... because I think it only lasts a little amount of time.

BUT if they're getting darker day by day, then I'd say you're coming up to O, unless you're pregnant (some people reckon OPKs can beused as pregnancy test LOL!). I'd say keep using them... even if you have to spend some money on more, it'd be worth knowing when O is...

Do you / have you had any EW or watery CM?