Women will be able to tell the exact moment they are most likely to conceive thanks to a new skin patch. The device measures the tiny changes in body temperature which indicate when ovulation occurs. It is far more accurate than any current system and could save the NHS - and individual couples - a fortune on unnecessary fertility treatment.

The patch, called DuoFertility, was developed by Cambridge Temperature Concepts Ltd, a firm started by PhD students at Cambridge University, and is available on the market since the second half of 2008. The patch consists of a small rubberized patch about the size of a coin, which women stick onto their skin, near their bra strap, with waterproof double-sided tape.

It stays on night and day, and records even the most minute changes in the woman's temperature, including the crucial body basal temperature, which can then be read using a sensor similar to a supermarket bar code scanner. The system gathers much more accurate data over the full 24 hours, and can "learn" about the wearer's personal cycles, becoming increasingly accurate after the first month of wearing.

Measuring Basil Body Temperature (BBT) allows an individual to know when they (or their significant other) is ovulating. The DuoFertility takes 20,000 more measurements of BBT then any other thermometer, thus increasing accuracy.

The results are displayed on the reader using colored lights, or it can be plugged into any computer using a USB port, and will display the information as a standard graph which can be printed off. It doesn't need any special software, and people could even plug it in in an internet cafe without any problem.

The monitor contains a new type of battery which uses very low power and will last up to eight months. The reader can be recharged using a standard USB cable. However, this won't come cheap, as the device retails for about 500 pounds sterling.