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This article presents the reply of the author and co-authors to an article about the analysis of differences in the fertility patterns between national populations and over time. In their comments to our paper Osona and Kohler make a number of interesting and potentially helpful points. While their comments do not, in general, appear to be in conflict with our opinions, we are concerned that they have misunderstood one specific point. In their first section, on the analytical relationship between Hadwiger parameters and demographic measures, the authors re-present some known results. Those results, of course, were not the essential point of our paper, which was concerned primarily with the analysis of differences in fertility patterns. The functions were fitted in order to gain understanding of the differences in the patterns between countries and periods. The correspondences between variables to which they refer were used in order to investigate the effects of departures from the models in cases of distorted distributions. In their section dealing with reparameterization the authors propose an alternative to our presentation of the Hadwiger mixture model. Their explicit interpretation of the Hadwiger parameters as the total fertility, the mean age at childbearing, and the variance of the fertility schedule is possibly a step forward for the use of the model in demography.

In this paper information collected in the World Fertility Survey on durations of breastfeeding and postpartum abstinence and amenorrhoea is used. Rates of conception and of resumption of menstruation are presented for four breastfeeding states: full breastfeeding, partial breastfeeding, weaned, and never breastfed. The various patterns of breastfeeding affect resumption of menstruation in consistent and expected directions. Continued breastfeeding beyond the resumption of menstruation and of sexual relations exerts a considerable contraceptive effect in all twelve countries considered, although interesting differences also emerge. The consequences of these findings are discussed. [ABSTRACT FROM AUTHOR]

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.

We focus on the fertility of Swedish men and women who lived in a consensual or marital union in the 1970s and 1980s, and where at least one of the partners had children before they entered that union. Couples without any children before the current union were included for contrast. We find clear evidence that couples wanted a shared biological child, essentially regardless of how many children (if any) they had before their current union. The shared child seems to have served to demonstrate commitment to the union, as did its conversion into a formal marriage. We have not found much support for the hypothesis that our respondents sought to enter parenthood to attain adult status. A second birth might have been valued because it provided a sibling for the first child -- a half-sibling acting as a substitute for a full sibling -- but our evidence for such effects is contradictory. Our analysis makes it very clear that parity progression depends on whose parity we consider. [ABSTRACT FROM AUTHOR]

Fecundability - the monthly probability of conception - incorporates both physiological and behavioural elements that vary across a woman's reproductive life span. How much of the variation in fecundability during the reproductive period can be attributed to age- related changes in physiology, and how much to variation in coital frequency? We use a mathematical model of fecundability to consider this question. Intra-uterine mortality has an important effect on fecundability: effective fecundability (the likelihood of a conception that results in a live birth) is less than half of total fecundability (the likelihood of any conception) at nearly all ages when coital frequency is held constant. The change in effective fecundability with increasing coital frequency is non-linear: it declines with increasing frequency. At all coital frequencies, the effects of increasing physiological age are greatest at the youngest and oldest reproductive ages, while between the ages of 20 and 30 physiological change has only a small impact on effective fecundability. [ABSTRACT FROM AUTHOR]

The article discusses the relationship between women's employment and marital fertility in the United States during 1970-1975. The relationship was found to change with the change of time and space. Fertility is detected as a cumulative process that takes place in discrete steps each time on the birth of a child. The fertility rate was depressing in case of working women due to the mechanism of competition for personal resources of women. The conception rate was also detected to be low during the months when women were working. Also, the preference for employment and preference for long birth intervals go together.

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.