Of the 11 kits tested by CR, only one proved to be sensitive enough to predict ovulation for nearly 9 out of 10 women.

The length of the LH surge varies from woman to woman, lasting between 10 and 31 hours in about half of women, less than 10 hours in 38 percent, and over 31 hours in 12 percent. In conjunction with an independent lab, Consumer Reports tested each model with urine spiked with various concentrations of LH. Following package instructions, LH concentrations were increased up to the point where a positive reading was obtained.

Frighteningly, Consumer Reports found "Though most of the kits are labeled as having 99 percent clinical accuracy, in actual use even the best-performing models are likely to detect LH surges in a considerably lower percentage of women. At their peak, most women's LH surges range from less than 20 to 100 mIU/ml.

The top-rated product, the ClearPlan Easy Ovulation, was the only one able to detect LH concentrations as low as 22mIU/ml. It was also easier to read than most other models. But even this product will not detect ovulation in the 12 percent of women whose LH peaks at below that concentration." The costlier ClearPlan Easy Fertility Monitor (the electronic tester) was the second most sensitive model tested, detecting LH concentrations as low as 36 mIU/ml, meaning it may work for about 65 percent of women.

"At the other end of the spectrum," CR says, "we had to spike the test urine with the highest LH concentration we tested - 91 mIU/ml - before we obtained a positive reading from the First Response pregnancy Planning Kit. Only 20percent to 25percent of women have an LH peak that high. Those results cast doubt on the product's label, which claims it contains enough test sticks 'for 90percent of ovulating women to detect hormone surge.'"

Amazingly, though, that wasn't even the worst-performing test Consumer Reports found. Answer Quick & Simple One-Step ovulation test got that dubious honor.

Ovulation tests