Among many of the castes of Hindustan, it is a religious ordinance that on the fourth day of menstruation a man shall have intercourse with his wife, “since this day is that on which conception is most likely to occur.” Indian physicians advise, in order to bring about conception, “that coitus be effected always as soon as the menstrual flow has ceased, at the end of the day, and when the lotus has closed.” In Japan, medical opinion is to the effect that a woman is capable of conceiving during the first ten days after menstruation, but not later (_Ploss_ and _Bartels_). The view that the first days of the intermenstrual interval are those most favourable to the occurrence of conception, is further confirmed by the statistical data collected by _Löwenfeld_, _Ahlfeld_, _Hecker_, and _Veit_; and it appears that as the date of the next menstruation is approached, there is a continual decline in the frequency of conception; just before the flow, conception hardly ever occurs. _Hensen_, from the records of 248 conceptions in which the date of the fruitful intercourse was exactly known, draws the following conclusions: 1. The greatest number of conceptions follow coitus effected during the first days after the cessation of the menstrual flow. 2. When coitus is effected during menstruation, the probability of conception increases day by day as the end of the flow is approached. 3. The number of conceptions following coitus effected shortly before menstruation is minimal. 4. However, there is no single day either of the menstrual flow or of the intermenstrual interval, on which the possibility of the occurrence of conception can be excluded. _Feokstitow_ has drawn up from statistical data an ideal “conception-curve,” which teaches that conception most readily ensues upon coitus effected soon after the end of the menstrual flow, in the first week, that is to say, of the intermenstrual interval; moreover, the curve shows that the highest percentage of conceptions occurs on the very first day after the cessation of the flow, and that after this day the percentage of conceptions declines. The percentage frequency of conceptions from coitus effected on the last day of menstruation, and on the first, ninth, eleventh, and twenty-third days, respectively, of the intermenstrual interval, is expressed by the ratio 48 : 62 : 13 : 9 : 1; and between the points given, the course of the curve is almost rectilinear. The probability of the occurrence of conception on the twenty-third day of the interval (on which day the curve reaches its lowest point), is one-sixty-second of the maximum probability.