Immediately prior to ovulation, the body produces a large amount of luteinizing hormone (LH) which triggers the release of a ripened egg from the ovary. This is known as an 'LH' surge. It is during this fertile time, that pregnancy is most likely to occur. A very useful test for couples trying for a baby, shown in laboratory trials to be more than 99.9% accurate. Note: this test should not be used a contraceptive method.