For the American media, marketers, and politicians alike, the Baby Boom has served as a kind of background radiation—a demographic version of the vestigial heat from the Big Bang, 2.7 degrees Kelvin, which bathes the cosmos. But the trend toward delayed childbearing is more properly the background radiation, with the Baby Boom flaring against it as an anomalous supernova. In Western nations the tendency for couples to marry later and have fewer children, at an older age, has been at work almost since the beginning of the Industrial Revolution.
As is by now commonly known, patterns of childbearing in the United States have shifted over the past three decades or so as women have entered the work force en masse. According to tables compiled by the National Center for Health Statistics, 66.7 percent of women aged twenty to twenty-four were childless in 1992, as compared with 47.5 percent of that age group in 1960, near the end of the Baby Boom years. More tellingly, 43.8 percent of women twenty-five to twenty-nine and 26.1 percent of women thirty to thirty-four were childless in 1992; in both cases this is almost double the corresponding percentage of those who were childless in 1960. Today roughly a quarter of all first-time births are to women thirty to forty-four years old, and the phrase "elderly prima gravida," traditionally employed by physicians to refer to women entering motherhood over thirty-five, sounds absurd in a whole new way.
The subject of delayed childbearing has filled many column inches in newspapers since the late 1950s and has all but dominated discussions of pregnancy in women's magazines, which no doubt shape as well as reflect social attitudes, and are influential dispensers of health information in any case. "CAREER BABIES," read a March, 1958, Mademoiselle headline; "Over 30? Over 35? Over 40? How Late Can You Wait To Have a Baby?" was an article that ran in the January, 1976, Ms.—just two examples among dozens of pieces appearing in Vogue, Harper's Bazaar, Glamour, Cosmopolitan, Good Housekeeping, McCall's, Redbook, Essence, Ebony, and Ladies 'Home Journal which dealt explicitly or implicitly with the phenomenon.
Such articles generally extolled the virtues of older motherhood and promulgated the myth that women can easily conceive any time until their early forties—maybe, considering those flukish change-of-life pregnancies, even up to menopause. In the 1960s women's magazines devoted considerable space to the increases in the risk of genetic defects, particularly Down syndrome, that come with advancing maternal age. But the advent of sonography, amniocentesis, and chorionic-villus sampling to gauge the health of the fetus tended to assuage worries, and coverage became almost universally upbeat. The intent was clearly to support women who put off having children in favor of furthering their educations and establishing careers, and to reassure those whose life circumstances had thus far worked against their desire to have children—for example, women who had not yet found a mate and were unwilling to undertake single parenthood. The author of a 1989 New York magazine article on women in their mid-forties having children expressed a fairly typical view in declaring of her subjects,
They are part of the first wave of baby-boomers, and they are intent on pushing old age back as far as they can push it. They are a tough, feisty, indomitable group of women who are redefining what it means to age in the most profound way they can—by having babies who will enter kindergarten after their mothers have celebrated their fiftieth birthdays.
Leaving aside all questions about the merits of older parenthood, and also the question of whether this putative indomitability might not also constitute a profound form of denial, such stories may be said to create the impression that any pre-menopausal woman who wishes to have a child may do so at will. A necessary corrective comes when one delves into the demographic literature.
Ever since the invention of statistics, at the turn of the nineteenth century, heads of nations have paid particular attention to the procreational habits of their citizens. Censuses have been taken, and marriage and birth certificates filed, so that statisticians in their bureaucratic warrens, tallying and manipulating the data in dozens of arcane ways, can sketch profiles of given populations. Deceptively dry-looking on the page, these columns of numbers and percentages, of figures broken out by age or race or sex or educational attainment, have often prompted paroxysms of national concern and dismay—as, for example, in the 1860s, when a rising birth rate in Germany in conjunction with a stationary one in France led to nervousness among the French, who believed that population growth bore a strong relationship to military might.
In attempting for predictive purposes to comprehend both the patterns underlying population fluctuations and the ways in which biological and societal forces interact to yield booms and busts, researchers began to distinguish between fertility, or the actual bearing of children, which is usually stated as a total number or rate per thousand women, and fecundity, or the capacity of a woman to conceive. (Medical scientists also speak of "infertilityz by which they may mean either the condition of childlessness or a difficulty in conceiving; in referring to the latter state, statisticians often prefer the term "impaired fecundity.")
Demographers have struggled to explain why younger women invariably register higher fertility than older ones. The obvious answer is that the female reproductive organs become less efficient with age, but establishing the truth of this is not as easy as one might think. To prove that the decline in fertility results from a deterioration of physiological function, one must separate out the effects of behavior from those of biology. The ideal for theoretical purposes would be a robust population in which breeding occurs without impediment over the entire reproductive lifetime of the female. Scientists have easily constructed such experiments with animals, and all the animal data support the hypothesis of an age-linked decline in fecundity. The situation is more complex with human beings, however. Modern populations do not in general approximate the required conditions, because surgical sterilization and contraceptive use are widespread. So demographers have turned instead to the study of small interbreeding groups that have not practiced contraception.
Perhaps the classic paper in this genre, by Joseph Eaton and Albert Mayer, sociologists from Wayne University, was published in a 1953 issue of Human Biology. Eaton and Mayer realized that records kept from 1880 to 1950 by the Hutterites, an Anabaptist sect living in scattered settlements in the Dakotas, Montana, and Canada, provided a means of carrying out "an ex post facto experiment in human biology." Close-knit, and meticulous about keeping their vital statistics, the Hutterites also observed a religious proscription against birth control. Eaton and Mayer saw that the Hutterites might help to answer the question "How many children can a group of human beings have if they reproduce up to the limit of their biological capacity and live through their entire period of potential fertility?"
The Hutterites proved to be extraordinarily fertile, registering a higher birth rate than any group previously examined. But more intriguing for demographers was a curve demonstrating that a Hutterite woman's chance of a live birth at a given age—her reproductive batting average, as it were—fell from just over .500 at age eighteen to .000 shortly after age forty-eight, with the curve taking a sharp bend southward at around age thirty-five. The Hutterite study became a standard reference, and its findings were bolstered in 1961 by a comparative survey of ten similar populations from around the world. But critics found possible reasons other than underlying biology for fertility drops: for example, sexual ennui—or, as they more technically put it, a diminution in coital frequency over the course of a marriage. The age of the male partner could also play a role, in part because the volume, vigor, and viability of sperm seem to begin to diminish around forty.
Demographers continue to debate interpretations of fertility figures and to question one another's methodologies and theoretical assumptions regarding the measurement of age-specific declines in fecundity. Yet although no consensus has emerged, statisticians such as William Mosher, who heads the Family Growth Survey Branch of the National Center for Health Statistics, and his colleague Anjani Chandra, who has written extensively on the demography of infertility, straightforwardly assert that "in women, fecundity decreases with age, particularly after the age of 30 or 35 years." Chandra says that one "could rage on forever about the actual critical age" at which fecundity sinks most sharply, but she thinks that virtually everyone agrees that the decline exists. "The truth of the matter," says James Trussell, an economist at Princeton University who is affiliated with the Office of Population Research there, "is that evolutionarily, women are designed to have children early. Of course, socially, things aren't designed that way."
Physicians, and gynecologists in particular, soon became aware of demographers' Sputnik-era discussions about fecundity, although the subject commanded only passing interest, because in those days infertility could not be overcome medically. Nonetheless, in the 1970s French medical researchers saw that they, like Eaton and Mayer, had been presented with a form of human biology experiment, an opportunity to arrive at a baseline reading of female fecundity.
Early on, French sperm banks and infertility centers had banded together under a single organizational umbrella. The Centres d'Etude et de Conservation du Sperme Humain, known by the acronym CECOS, had adopted standardized approaches to freezing and handling sperm and to treating patients. Around 1973 several CECOS researchers realized that by performing artificial insemination with donated sperm on a group of reproductively healthy but childless women of mixed ages who had come to the centers because their husbands were sterile, they could arrive at an approximation of "natural" fecundity. Artificial insemination under controlled conditions would eliminate confounding factors such as variations in coital frequency or in sperm quality.
By the time the CECOS clinicians wrote up their results, they had followed 2,193 women over the course of seven years, 1973 to 1980, and had observed a "slight but significant" decrease in the ability to conceive after age thirty and a "marked" decrease after age thirty-five. Whereas women under thirty-one had about a 74 percent chance of being impregnated within a year's time, the figure fell to 61 percent for those aged thirty-one to thirty-five, and to 54 percent for those over thirty-five. These were the numbers that prompted DeCherney and Berkowitz's New England Journal of Medicine editorial on the need to rethink delayed childbearing.