Symmetry and ovulation in women.

Women have cryptic ovulation, and self-observation methods of determining the timing of ovulation, such as monitoring cervical mucus symptoms or recording basal body temperature, are not reliable. It has recently become apparent that the symmetry of paired soft tissue traits, such as breast size and digit length, changes across the menstrual cycle. This paper presents evidence that symmetry in four paired soft tissue traits showed a marked increase on the day of ovulation. The difference (i.e. the asymmetry) between the size of the left and right trait in ears, 3rd, 4th and 5th digits was measured. The timing of ovulation was confirmed by real-time pelvic ultrasonography and trait measurements were made without knowledge of scan results. Asymmetry was lowest on the day of ovulation (day 0), decreasing by about 30% from day -1, and significant within-subjects changes occurred from days -2 to day -1, and day -1 to day 0. The practical and evolutionary implications of these findings are discussed.