Fertility, Proximate Determinants of
FERTILITY, PROXIMATE DETERMINANTS OF
The proximate determinants of fertility are the biological and behavioral factors through which the indirect determinants–social, economic, psychological, and environmental variables–affect fertility. The distinguishing feature of a proximate determinant is its direct connection with fertility. If a proximate determinant, such as contraceptive use, changes, then fertility necessarily changes also (assuming the other proximate determinants remain constant), while this is not necessarily true for an indirect determinant of fertility such as income or education. Consequently, fertility differences among populations and trends in fertility over time can always be traced to variations in one or more of the proximate determinants. The following simple sequence summarizes the relationships among the determinants of fertility.
These relationships were first recognized in the mid-1950s when Kingsley Davis and Judith Blake defined a set of proximate determinants that they called the "intermediate fertility variables." John Bongaarts and Robert C. Potter defined a somewhat different set of proximate determinates in the late 1970s and early 1980s, greatly simplifying the task of constructing
models of human reproduction. This set will be presented here.
Proximate determinants are easily identified by examining the events that most immediately affect the duration of the reproductive period and the rate of childbearing during that period (see Figure 1). The potential reproductive years start at menarche, a woman's first menstruation. Actual childbearing in virtually all societies, however, is largely limited to women in socially-accepted sexual unions such as marriage. Since, with few exceptions, first cohabitation takes place some time after menarche, this delay reduces the number of years available for reproduction. If exposure to the possibility of conception is maintained, childbearing can continue until the onset of permanent sterility, which takes place at or before menopause.
While in a sexual union and fecund, women reproduce at a rate inversely related to the duration of the birth interval: short birth intervals are associated with a high birth rate and vice versa. In the absence of intrauterine mortality, the duration of a birth interval is determined by three time components. The first is the postpartum infecundable interval from birth to the first postpartum ovulation. During this period women are unable to conceive. The second is the fecundable interval (also called the ovulatory interval, or the waiting time to conception) from the first ovulation to conception. During this period women are able to conceive if they have sexual intercourse. The duration of this interval is determined by the monthly probability of conception, which is in turn determined by the level of fecundability (the monthly conception probability in the absence of contraception) and by the effectiveness of any contraception practiced. The third time component is a full-term pregnancy interval of nine months. In the event that an intrauterine death occurs, whether spontaneous or induced, the birth interval is lengthened by additional components: the shortened pregnancy is followed by a very brief infecundable period and an additional fecundable period (Figure 1).
This overview of the reproductive process identifies the following proximate determinants:
onset of cohabitation and union disruption;
onset of permanent sterility;
the duration of postpartum infecundability;
use and effectiveness of contraception;
spontaneous intrauterine mortality; and
The first two of these factors determine the duration of the reproductive period, and the latter five determine the rate of childbearing.
Onset of Cohabitation and Union Disruption
In recent decades cohabitation before marriage or without marriage has become increasingly common, but in most societies marriage remains the main form of socially-sanctioned cohabitation. The mean age for women at first marriage varies widely among populations. In traditional societies in Asia and Africa, first marriage often takes place relatively soon after menarche. In contrast, the mean age at first marriage or cohabitation in a number of European populations is near 25 years. The timing of first marriage is correlated with the prevalence of permanent celibacy; populations with a high age at marriage tend to have high proportions of women who never enter a union and vice versa.
Patterns of union disruption have changed considerably over time. While divorce was historically uncommon worldwide until recent decades, it has increased rapidly in developed countries. In the developed world, a relatively large proportion (in a few cases more than one-third) of all marriages end in divorce. The fertility impact of divorce is minimized by the rapid remarriage of the majority of divorced women of reproductive age. Widowhood was historically an important cause of union disruption in all countries, but its prevalence has declined with the level of mortality. For example, in India in 1901, 46 percent of women had been widowed by age 45. In contrast, in the twenty-first century only a few percent of women in the developed world experience widowhood during their reproductive years.
The mean age at onset of cohabitation, the prevalence of permanent celibacy, and the rate of union disruption are the main determinants of the average proportion of reproductive years women are exposed to the possibility of childbearing. In populations with early and universal cohabitation, the proportion of the potential reproductive years lost is typically one-fifth or less, but this proportion can approach one-half in populations with late onset of cohabitation and a high incidence of permanent celibacy.
Onset of Sterility
Menopause, the complete cessation of menstruation, marks the end of the potential childbearing years. In the United States and western Europe, an individual woman's age at menopause can range from less than 40 to near 60, with averages around 50. Only a few studies have been made in developing countries, and because of various methodological problems including recall errors and age misreporting, it is not clear whether a substantial difference in mean age of menopause exists between the populations of developing and developed countries.
Postmenopausal women are definitely sterile, but the onset of sterility can occur several years before menopause. Menstrual cycles become increasingly irregular in the years before menopause, presumably reflecting a rising incidence of anovulatory cycles. A high risk of spontaneous intrauterine mortality also contributes to reduced fecundity among women over age 40. In addition to these sterility factors for women, there is some sterility (but of lower frequency) among their male partners. The resulting couple sterility is estimated to reach 50 percent when women are in their early forties. This early age of onset of sterility is consistent with observations of a mean age at last birth of around 40 years in many populations that do not practice contraception.
The duration of the anovulatory interval after a birth is usually estimated from the delay in the return of menstruation–the interval of postpartum infecundability is assumed to equal the duration of postpartum amenorrhea. This assumption is apparently quite accurate when applied to the average interval in a population, although in some women the first ovulation precedes the first menstruation. It is now well established that the duration and pattern of breastfeeding are the principal determinants of the duration of postpartum amenorrhea. In the absence of breastfeeding the menses return shortly after birth, with average amenorrhea durations of 1.5 to 2 months. As the duration of breastfeeding increases, so does the amenorrhea interval. A woman experiences approximately one additional month of amenorrhea for each two-month increment in breastfeeding duration. With long lactation, mean amenorrhea intervals from one to two years are observed in developed as well as in developing countries. Several studies, comparing entire populations or subpopulations between countries, have documented high levels of correlation between breastfeeding and amenorrhea durations. On the individual level the correlation between lactation and amenorrhea intervals, while still highly significant, is somewhat lower. The most plausible explanation for this, aside from measurement error, is that women differ not only with respect to the duration of breastfeeding, but also with respect to the type and pattern of breastfeeding. Women who fully breastfeed have a lower probability of resumption of menses than women whose infants receive supplemental food such as fluids by bottle or solids. The inhibiting effect of breastfeeding on ovulation and menstruation, as well as the differential impact according to the type and pattern of breastfeeding, are believed to be the result of a neurally-mediated, hormonal reflex system stimulated by the child's sucking the breast nipple.
Fecundability equals the monthly probability of conceiving among women who menstruate regularly but do not practice contraception. Typical average fecundability levels among young cohabiting partners range from 0.15 to 0.25, depending primarily on frequency of intercourse. Lower values are found at higher ages and longer durations of cohabitation. This monthly conception probability is substantially less than 1.0 because fertilization can only take place during a short period–approximately two days–around the time of ovulation in the middle of a menstrual cycle. In addition, some cycles are anovulatory and a substantial proportion (perhaps a third) of fertilized ova fail to implant, or are spontaneously aborted in the first two weeks after fertilization. These aborted fertilizations are usually not counted as conceptions in the demographic literature because they cause little or no disruption in the menstrual cycles and women are often unaware of such brief pregnancies. Levels of fecundability around 0.2 imply that many women do not conceive for a number of months even if they have regular intercourse. Typical average delays to conception range from five to ten months.
The prevalence of contraception varies widely among populations. The percent of cohabiting women of reproductive age currently using contraception ranges from near zero in a number of developing countries with high fertility, to above 75 in a number of developed countries. The use of contraception affects fertility because it decreases the risk of conception. The effectiveness of contraception is measured as the percent of reduction in fecundability. For example, a contraceptive with an effectiveness of 90 percent used by a group of women with a fecundability of 0.2 will yield an actual monthly probability of conception of 0.02. Contraceptive effectiveness depends on the method as well as on the motivation and knowledge of the user. In developed countries, the effectiveness of modern methods such as the birth control pill and the intrauterine device (IUD) is more than 95 percent, and the effectiveness of conventional methods such as the condom, diaphragm, or spermicides is around 90 percent. Those levels are believed to be lower in developing countries, but reliable information about effectiveness in these populations is virtually nonexistent.
Spontaneous Intrauterine Mortality
It has proven difficult to make estimates of the risk of intrauterine mortality. Retrospective reports of pregnancy histories of individuals are known to be deficient because of recall errors, but estimates based on prospective studies also vary. This is in large part due to the difficulty in obtaining accurate reporting of intrauterine deaths in the early months of pregnancy, when it may not be easy to distinguish between a delayed menstruation and an early spontaneous abortion. The most carefully designed studies estimate that about 20 percent of conceptions will not end in a live birth (not including embryonic deaths occurring before the first missed menstruation). Nearly half of these spontaneous abortions occur before the third month of pregnancy. This estimate of 20 percent is an average for women of all ages. The risk of intrauterine mortality is lowest in the mid-reproductive period and much higher than average for women in their late thirties and forties. The available evidence does not suggest large differences
in the risk among societies. However, the probability of a stillbirth (an intrauterine death after the 28th week of gestation) is around 4 percent of conceptions in some poor countries, while it is only about 1 percent in the most developed countries. The reasons for this difference have not been determined conclusively, but health and environmental factors presumably play an important role.
Deliberate interventions to terminate pregnancies have been practiced throughout recorded history. In the mid-1990s, the proportion of pregnancies ended by induced abortion ranged from near zero in some countries to more than one-half in some parts of Eastern Europe. The availability of simpler medical techniques, assurance of personal safety, and ease of access have recently increased in many countries. Even where these conditions are not present, as in much of the developing world, the determination to avoid childbirth may lead women to resort to induced abortion.
Analyzing Fertility Levels
Each of these seven proximate determinants directly influences fertility, and together they determine the level of fertility. In studies of fertility levels or differentials it is generally not necessary to devote the same effort to analyzing and measuring each of the proximate determinants because they are not of equal interest. Two criteria can be applied to select the proximate determinants that deserve most attention. The first is the sensitivity of the fertility rate to variation in a determinant; it is relatively uninteresting if large variation produces only a minor change in fertility. The second criterion is the extent of a determinant's variability among populations or over time. A relatively stable determinant can contribute little to explaining either trends or differentials.
In Table 1, the seven proximate determinants are given an approximate rating for these two criteria. Studies with reproductive models (such as the 1983 study by Bongaarts and Potter) show that fertility is least sensitive to variations in the risk of spontaneous intrauterine mortality, and most sensitive to changes in the proportions of women in union and the prevalence of contraception. Variability is lowest for onset of sterility and risk of spontaneous intrauterine mortality. The overall rating, based on both criteria, indicates that four proximate determinants–onset of cohabitation, postpartum infecundability, contraception, and induced abortion–are the most important for the analysis of fertility levels and trends.
Bongaarts, John. 1978. "A Framework for Analyzing the Proximate Determinants of Fertility." Population and Development Review 4(1): 105–132.
Bongaarts, John, and Robert C. Potter. 1983. Fertility, Biology, and Behavior: An Analysis of the Proximate Determinants. New York: Academic Press.
Davis, Kingsley, and Judith Blake. 1956. "Social Structure and Fertility: An Analytic Framework." Economic Development and Cultural Change 4(3): 211–235.
Stover, John. 1998. "Revising the Proximate Determinants of Fertility Framework: What Have We Learned in the Past 20 Years?" Studies in Family Planning 29(3): 255–267.
Wood, James W. 1994. Dynamics of Human Reproduction: Biology, Biometry, Demography. New York: Aldine de Gruyter.