I have been taking part in a clinical trial of a "fertility meter": this involves peeing on lots of sticks, and trying to persuade Mr Spouse into bed at the appropriate time of the month. There are three groups in the trial: one gets information about when they have an LH surge (this is what the ovulation predictor kits you buy at the chemists' measure), one gets no information (I have ranted on various message boards about why women in this group should jolly well stay in the study and not go off in a huff), and one, my group, gets information about the oestrogen surge that comes before the LH surge.
Now typically this surge lasts a couple of days (I've only ever had ones lasting either one or two days), so given my pretty regular cycles, this means I usually get a "high" (i.e. "now would be a good time to jump on your husband") reading around day 12 and 13, with reversion to low around 14 or 15, and that means my period would come two weeks after that. The first of the low readings after a high are actually when a normal ovulation thingy would read "fertile".
So it's a very good way to make me extremely paranoid (has my body given up? am I going to have a three week cycle? am I not going to ovulate? is this the end?) when on day SIX I see a high reading. Although in theory, what this should mean is that I inveigle Mr Spouse into bed about every other day for a week or so before, as well as during, the most fertile time, which is actually a method that seems to work just as well as targeting the most fertile time [in fact, that is why this meter is only being trialed - though I don't doubt it will go on sale, as desperate women will probably pay loads for it].
But in practice what happens is that I'm feeling so paranoid that this rubs off on Mr Spouse, putting neither of us in the mood.