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.

With regard to the probability of pregnancy from intercourse on days relative to ovulation, ultrasound and hormonal studies have found that these are 6 days during the menstrual cycle when it is possible for a woman to become pregnant.
Data on the probability of pregnancy on different days around ovulation indicates that:
  • 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.
These probabilities are due to the limited viable life span of the sperm after ejaculation (not more than 5 days) and to the very limited viable life span of the egg following ovulation (less than 24 hours).
Together these result in an actual fertile window of no more than 6 days during the woman’s cycle.  On all the other days, the woman cannot become pregnant.
Putting this in practical terms, if a woman has sex on Saturday and ovulates the following Wednesday, she has a 10 – 12% chance of getting pregnant that cycle.  But if she has sex on Saturday and doesn’t ovulate until the following Friday, she has a basically zero % chance of pregnancy.
But how do we know the moment ovulation occurs, and thus know the exact fertile window?
Again, consider probabilities.  Data on when during the cycle ovulation occurs indicates that in the great majority of cycles, ovulation occurs very close to the middle of the cycle.
 
The Institute has followed a research-to-practice model in the development of the Standard Days Method® (SDM) and has collaborated with a wide range of research partners in this process. After establishing the theoretical basis of the method, it was tested in a multi-country efficacy study. Following the successful results of this study, the method was introduced on a small scale through pilot studies around the world. Studies on different service delivery strategies, long-term follow up studies, and larger-scale integration studies followed. This research led to establishment of best practices to provide SDM and development of guides and simple tools to help organizations incorporate the method into programs and services. Research on scaling-up efforts and new service delivery strategies is currently underway under the FAM Project.

 

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