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The paper deals only with the first two of these, which, unlike duration of pregnancy, present unusual problems of estimation. The literature of the past few years contains a number of attempts to deduce values for these determinants from observational data, that is, from observed series of birth intervals, but it is one of the more frustrating aspects of such demographic analyses that the significance of observations often depends as much upon the way in which the data were collected as on the values themselves. Sets of birth intervals can be obtained in a quite remarkable variety of ways and analyzed with correspondingly varied virtuosity. The fecundabilities calculated in various tables are considerably lower than those, which have been derived from other studies. This is largely because of the inclusion in the series of the full range of women in the population; the correction of the harmonic effect also leads to a lower value. A remarkable feature of this series is that some 4% of all intervals were stated as being longer than six years. Intervals of this length are, of course, very much rarer in studies confined to women of high attained parity.

This article describes a simple study of the chance of conception assuming a fixed-length menstrual cycle, a specific probability distribution for the day of ovulation, and a broad and a narrow fertile period. Eight different patterns of intercourse throughout the cycle are considered with frequency of intercourse ranging between 5 and 12 times per cycle. Results are given in terms of chance of conception and in terms of waiting times until conception. A number of factors contribute to the probability of conception: the frequency and timing of intercourse, the viability of the sperm and ovum, and the total sperm count among others. The length of the menstrual cycle is far from constant, and the distribution of time of ovulation is at best a rough approximation to reality. In addition, the menstrual period does not have a constant duration of four days as assumed in the model. An assumption, implicit in this approach, has been that heightened coital frequency does not reduce the sperm count enough to affect virility. These problems could probably be handled satisfactorily using available information.

The article focuses on birth interval dynamics. A birth interval free from pregnancy wastage may be viewed as the sum of three sub-intervals namely the period of post-partum amenorrhoea following a birth, the menstruating interval extending from the end of post-partum amenorrhoea to the next conception and the months of pregnancy leading to the next live birth. When the birth interval includes a pregnancy loss, then in addition to the above three components there is a fourth component, which will be termed time added by pregnancy wastage. This latter consists of the pregnancy leading to wastage, a month or more of post-abortion or post-stillbirth amenorrhoea, and a second menstruating interval. In a small minority of birth intervals the length of this fourth component is increased by the occurrence of two or more foetal losses. A basic understanding of birth intervals requires the ability to relate the central tendencies and variability of birth intervals to the means and variances of these four components.