Based on sophisticated computer modelling of reproductive physiology data, SDM identifies the 12-day "fertile window" of a woman’s menstrual cycle. These 12 days take into account the life span of the woman’s egg (about 24 hours) and the viable life of sperm (about five days) as well as the variation in the actual timing of ovulation from one cycle to the next. The study found the efficacy of the SDM to be comparable to or better than a number of other widely-used methods of family planning, such as the diaphragm and the condom.
"For millions of women worldwide who rely on periodic abstinence as their primary form of family planning but do not have a clear understanding of which days in their cycle they should avoid unprotected sex, SDM is a useful option," say the researchers. To keep track of which days to avoid unprotected intercourse, the women used a string of 32 colour-coded beads with each bead representing a day of the menstrual cycle. Beginning with the red bead, which represents the first day of her menstrual period, the woman moves a small rubber ring one bead each day. The brown beads are the days when pregnancy is very unlikely, and the glow-in-the-dark white beads (beads 8-19) represent her fertile days. The method works best for women whose cycles are usually between 26 and 32 days long.