The life span of spermatozoa inside mother´s delivery tract is short and also the egg cell has to get fertilised within 24 hours after release from the ovary. The release of the egg cell from the ovary is preceded by a surge of luteinising hormone (LH) from the hypophysis. This LH surge is of short duration, but can be detected from urine sample by ovulation tests.The LH surge indicates that the egg cell will be released within 24-36 hours. Thus, this time frame is the best time for coitus to enable fertilisation of the egg cell followed by attachment to uterus.
To notice the LH peak, the test is usually made in five successive days covering the days in the middle of the menstruation cycle. The starting day depends on the length and regularity of the menstruation cycle. For each individual type of menstruation cycle, the starting day of LH testing is given in the table supplied with the test.