The two most widely-used fertility monitors are both considerably more expensive than other ovulation prediction tools, costing between $160 and $300 for the machines themselves, plus up to an additional $50 for 30 urine test sticks to be used in the ClearBlue Easy Fertility Monitor.

However, proponents of these re-usable machines say that money is saved in the long run, since many women do not get pregnant in their first few months of trying to conceive and they could virtually spend as much money on disposable ovulation prediction products.

The ClearBlue Easy Fertility Monitor relies on disposable urine sticks to detect varying levels of estrogen and luteinizing hormone.

The other, the OvaCue Fertility Monitor, uses a plastic sensor and wand to collect either saliva or cervical fluid and detect electrolytes such as sodium and potassium.

Both of these monitors guide the user as to when to test, based not only on the initial information she puts into the machine's computer, but also on any personal patterns detected by previous uses of the monitor. Both products aim to provide a woman with a larger window of fertile opportunity for conception during each cycle.

Successful users of these monitors may be interested to know that used fertility monitors are sometimes re-sellable via online auction or other similar services.