Scientific Basis of the SDM
Standard Days Method® (SDM) is based on reproductive physiology. Most ovulations occur around the mid-point of the menstrual cycle (+/- 3 days). Thus a woman's fertile "window" (days in the menstrual cycle when she can get pregnant) begins approximately five days prior to ovulation and lasts up to 24 hours after ovulation. This is because the sperm remain viable in the woman's reproductive tract for up to five days, and the ovum can be fertilized for up to 24 hours following ovulation.
Researchers at the Institute for Reproductive Health (IRH) identified the fertile window in the woman's menstrual cycle, using a computer simulation that took into account the probablility of pregnancy, probability of ovulation occurring on different cycle days, and variability in cycle length from woman to woman and from cycle to cycle. Their analysis found that avoiding unprotected sex on days 8 through 19 of the cycle provided maximum protection from pregnancy while minimizing the number of days to avoid intercourse. Women with menstrual cycles between 26 and 32 days long can use SDM to prevent pregnancy by avoiding unprotected intercourse during the 12 fertile days identified by the method.
- There is approximately a 4% probability of pregnancy from intercourse 5 days before ovulation.
- This increases to 15%, 4 days before ovulation.
- The highest probability of pregnancy – between 25 and 28% -- is on the 2 days before ovulation.
- On the day of ovulation, there is an 8-10% probability.
- Fertility then decreases – with a 0% probability of pregnancy by the day after ovulation.