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Home ovulation test, positive result. Test stick used to test urine for a hormonal surge that marks the onset of ovulation. Two lines (blue) appear in the result window. As the surge line (left) is darker than the reference line (right), the result is positive. Ovulation is the release of an egg (female reproductive cell) from an ovary. Because the egg only survives for 24 hours, detecting the start of ovulation aids planning for a pregnancy. The test stick has an absorbent tip (not seen) for dipping in a urine sample. The hormone detected is the luteinizing hormone secreted by the anterior pituitary gland, stimulating ovulation in women. (KEYSTONE/SCIENCE PHOTO LIBRARY/CORDELIA MOLLOY)