A.L.
hi
they work by measuring your LH (lutenizing hormone) which technically rises (when all things are a go) right before you ovulate.. hence sending a signal to your brain... release an egg.. with that said.... NOT all kits can necessarily detect the LH surge in that some women's surge is too low for the kit's threshold. Also, keep in mind a kit does not detect IF you are really ovulating but simply gages the LH surge.. OFTEN woman confuse this fact .. how can you truly know if you ovulate? charting is a good method.. whereby you check different things one in particular is your basal body temp.. when you have a surge in body temp mid cycle (whatever your midcycle is for you as not all are 14 days) and IF the temp stays about a baseline for approx 3 days, it is usually indicative of your having ovulating.. (See how to take charge of your fertility) it's a great book. Ovulation kits work in conjuction with taking your temp and other bodily signs and yes, if the LH is present can alert you to the fact that having sx within the next 24 to 48 hrs would be a good time to try and conceive..
They can get expensive though.. 12 to 15 dollars with only 7 in a box..... try the strips.. they are MUCH cheaper and you may have to buy them online....
depending how long you need to use them.. it's the way to go.... the boxed ones add up in price..
on fertilityties.com there is a store link.. they sale them on there much cheaper..
best of luck