Women trying to conceive are better served by a urine test to determine if they are ovulating than the more commonly used calendar method, the makers of the test said at a Stockholm conference Tuesday. \"The calendar method is good to help women to start to understand how their cycle works, but if women are really trying to conceive ... it\'s not really the best method to use,\" said Jayne Ellis, head of scientific and medical affairs at SPD Swiss Precision Diagnostics, which makes the Clearblue ovulation test. According to a study conducted by the company, the calendar method predicted ovulation correctly in only one in four women, while the test \"predicted correctly in 99 percent of women over the same period,\" the authors of the study said in a statement. The calendar method, which uses the previous cycle length and subtracts 14 or 15 days to give an estimate of the day of ovulation, is used by about 35 percent of women, the researchers found, pointing out that many websites and mobile phone applications offer calculation assistance. Ellis and her team had asked a group of 101 women to collect daily urine samples for a total of 895 cycles, and then compared the accuracy with which the calendar method and the Clearblue test predicted peak fertile days. \"We found that the calendar method was inaccurate in predicting ovulation and therefore the peak fertile days,\" Ellis said in the statement, adding that \"this is because it uses data from previous cycles which are naturally variable in many women.\" The ovulation test, which like a simple pregnancy test comes in the shape of a pen, can when the tip is stuck into the urine flow detect a surge in luteinising hormones (LH) which trigger ovulation. When using a 20-stick pack of tests, women detected their peak fertile days in 99 percent of the cases, the study showed. \"It really pinpoints for women the most fertile days to conceive in their cycle,\" Ellis told AFP. The research team referred in its statement to previous studies showing that 46 percent of cycles in women between 18 and 40 vary by seven days or more, and that variations increase when women approach menopause. \"At a time when more and more women are delaying pregnancy until their 30s, it is increasingly important that they have a better understanding of their own menstrual cycles and the days on which they are fertile,\" the statement said.