Child Spacing: The Mathematical Probabilities

The chances of spacing children by the rhythm method are experimentally and theoretically analysed. Let q be the probability that a nonpregnant fertile female can go through 1 specified monthly cycle without conceiving. If there is no coitus during this cycle then q is 1; if there is coitus q is less than 1. p is taken from Potters equation and defined as the monthly risk of pregnancy; q then is equal to 1-p. It was shown that to achieve an even chance of spacing the birth of the next child 2 1/2 years from the birth of the last one the woman must secure a q of 97% of better. It is estimated that one needs 93% for the monthly security factor q with the rhythm method; this is consistent with about 3-4 acts of coitus per cycle if the fertile period is only 12 hours. However use of The rhythm method depends on close observation of a number of cycles and most couples practising this method cannot go through such observation without encountering pregnancy first. Therefore this method is likely to fail with many couples particularly couples with more children since the menstrual pattern becomes more and more changeable with repeated pregnancies. What is needed is an accurate ovulation-prediction test and research into the natural temporary sterility of the male partner.