THE makers of a hand held hormone monitor claim that it will tell women when love making will lead to pregnancy and when it will not.
The device shows a green or red light, indicating a woman's hormone level and fertility. It reads the likelihood of ovulation from a disposable urine test stick.
Persona, a "revolution in contraception" is 95 percent reliable - about the same as condoms - Unilever, the maker, said.
The monitor will display the green light on the days a woman is not likely to get pregnant by having sex and a red light shows that she is at risk of becoming pregnant.
For the first monthly cycle, she will be asked for 16 early morning tests. These are used to build up a picture of her individual cycle. The user needs to perform eight tests a month and can expect between six to 10 red light days in each cycle.
Prof Bob Snowden, of the University of Exeter, undertook a large scale behavioural study to assess the new method.
He said: "We are very encouraged by the study results. They show that it is very simple to use; 95 per cent of women found Persona very easy to use.
"Other feedback from the study was that more than half of the couples said that using it had improved their sex lives."