Women and men in many countries still lack adequate access to contraceptives. Unless they are given the option of controlling their fertility, severe environmental and health problems loom in the coming century throughout large parts of the world During 1999, the world's population surged past the six-billion mark. The most recent billion was added in just 12 years. Such numerical milestones, like this month's calendrical rollover, are of course just arbitrary artifacts of our decimal counting system, yet they offer a suitable occasion for taking stock of important trends.

Worldwide, the average number of children born to each woman-the fertility rate-has declined over the past three decades, from almost six to 2.9, prompting some commentators to venture that overpopulation may no longer be a threat. They are mistaken. Global population is still increasing by about 78 million people--a number equivalent to a new Germany-each year. Moreover, because large families were common in most of the world until recently, many countries have very large numbers of young people.

This population structure means that rapid growth is sure to continue for decades to come, almost all of it in developing countries, where family-planning services may be deficient or nonexistent. In nations that lack adequate medical, financial and educational institutions, not to mention food and water supplies, the result of a fast-growing population is much human misery. The quality of life of a large proportion of humanity during the coming century--and the future size of the global population--will depend critically on how quickly the world can satisfy the currently unmet demand for family planning.

Every day more than 400,000 conceptions take place around the world. Almost half are deliberate, happy decisions, but half are unintended, and many of these are bitterly regretted. A series of surveys in over 50 low-income countries has asked more than 300,000 women how many children they want to have. In nearly every country surveyed, women are bearing more offspring than they intend. When I practiced obstetrics in a London hospital in the 1960s, I would ask new mothers, "When do you want your next baby?" Many replied, "Doctor, I was just going to ask you about that." They were glad, in other words, that I had opened the door to an embarrassing but important topic. My boss in the hospital, however, berated me for discussing birth control. I learned that family planning was wanted but controversial.

During the past 30 years, many countries have greatly improved their provision of family-planning services. Contraceptive use in the developing world has risen from one in 10 couples to more than half of all couples. A 15 percent increase in the use of contraceptives means, on average, about one fewer birth per woman. Thus, in Ethiopia only 4 percent of women use contraception and the fertility rate is seven, while in South Africa 53 percent use some method and average fertility is 3.3. The desire for smaller families is spreading. In 1998 researchers associated with the Asian Development Bank in Laos, one of the world's poorest countries, invited people there to say what help they wanted most. The men requested jobs, but the women's number-one priority was family planning.

The unmet need for contraceptives is clearly on a different scale in Ethiopia or West Africa, where women commonly bear six children, than in, say, Italy, which has one of the lowest fertility rates in the world- 1.2. Yet wherever people have said they want fewer children and family planning has been made available, fertility has fallen. What they need is access to a variety of methods, backed up by safe abortion if they choose it. The pill, the condom and injectables are the types most likely to be widely useful in developing countries.

The trouble is that in some parts of the world contraceptives are either too expensive or simply unavailable to the people who most need them. The female condom, a recent development, may prove too costly for use in the most impoverished regions. I have seen women in Sri Lanka who were eager to control their fertility but so poor that they had to buy oral contraceptive tablets five at a time rather than in a monthly pack of 21. An estimated 120 million couples in developing countries do not want another child soon but have no access to family-planning methods or have insufficient information on the topic. Consequently, pregnancy too often brings despair instead of joy.

Limiting family size can be difficult. A healthy woman may be fertile between the ages of 12 and 50, and men produce viable sperm from puberty until death. Many couples engage in intercourse without taking precautions because they cannot find or afford contraception. For others, sex can be a violent act that leaves a woman with no opportunity to protect herself against unwanted pregnancy. A survey conducted in 1998 in the Indian state of Uttar Pradesh found that 43 percent of wives had been beaten by their husbands. If such women are to be helped, contraceptives have to be very easy to get.

In many countries, laws create hurdles. Japanese women were until this past year forbidden access to the pill and so had to rely heavily on abortion. Until the early 1990s, condom sales in Ireland were restricted to certain outlets, and even today some pharmacists refuse to sell them. The Indian government does not allow injectable contraceptives to be used, although the method has proved popular in neighboring Bangladesh. The rich typically have ways to get around such obstacles, but the poor do not.

In some nations, contraceptives are available only by medical prescription. This means that they cannot reach the many villages in Asia and Africa where there are few or no doctors. In Thailand, large numbers of women started to use birth-control pills as soon as nurses and midwives were given the authority to distribute them. Restrictive medical practices limit family-planning choices and make contraception more expensive but add nothing to safety. Birth-control pills are safer than aspirin. The world would be a healthier place if oral contraceptives were available in every corner store and cigarettes were limited to prescription use.

Changes in South Korea and the Philippines present a stark example of how family size plummets when consumers are offered a range of appropriately priced contraceptive options through convenient channels. In 1960 families in both countries had an average of about six children. By 1998 fertility had fallen to 1.7 in South Korea. In the Philippines, though, fertility was still 3.7, because family-planning help is harder to get there. Economic research strongly suggests that small family size is a prerequisite to higher per capita income. The difference in fertility rates between South Korea and the Philippines thus probably goes a long way toward explaining why income in South Korea reached $10,550 per person in 1998, whereas in the Philippines it was only $1,200.

In Colombia, fertility fell from 6 to 3.5 in only 15 years after contraceptives became widely available in 1968. In Thailand the same jump took a mere eight years. That identical transition took the U.S. almost 60 years, from 1842 to 1900: anti-vice activist Anthony Comstock persuaded Congress to restrict sales of contraceptives in 1873, and it was not until 1965 that the Supreme Court struck down the last laws banning contraception. No surveys of desired family size were conducted in the U.S. in the 19th century, but I suspect that many couples had more children than they intended.

The contrasting cases of Bangladesh and Pakistan illustrate particularly well how family planning can help women escape centuries of obedience to their mothers-in-law and of subservience to their husbands. Until a civil war in 1971, these two countries were a single political unit, and women had an average of seven births. Over the past 20 years, Bangladesh has made a systematic effort to provide a variety of fertility-regulation methods, including the pill and injectables. With these, women can control whether or not they become pregnant--an advantage they may lack if they rely on their husband's use of a condom. As a consequence, in spite of appalling poverty, fertility has fallen to 3.3 as contraceptive use among Bangladeshi women has risen from 5 percent in the 1970s to 42 percent today. Similar changes have not occurred in Pakistan, where most of the population still does not have access to fertility regulation, and women there bear an average of 5.3 children. These differences will have consequences that will last well into the 21st century. Although Bangladesh will increase its numbers by 65 percent by 2050, Pakistan will probably by then have reached 2.2 times as many people as it has today.

My lifetime has seen the most far-reaching demographic changes in history. Global population has almost tripled since I was born in 1935; it has quadrupled during the past century. The primary reason is a welcome decline in infant and child mortality brought about by the spread of public health measures such as vaccination. Unfortunately, this progress has not been accompanied by a parallel spread of modern contraception.

It is only since the 19th century that families have routinely seen more than two children survive to the next generation-otherwise there would have been a population explosion centuries ago. Large families are a recent, and temporary, anomaly. Small families reduce stress on the environment, benefit economies--and gain directly themselves. Research in Thailand has shown that children born into families with two or fewer offspring are more likely to enter and stay in school than are children from larger families of four or more youngsters. When pregnancies are spaced at least two years apart, both mother and baby are significantly more likely to survive. Worldwide, one woman dies every minute as a consequence of pregnancy, childbirth or abortion. Some 99 percent of these deaths are in developing countries. Better access to contraception would reduce this toll substantially by saving on the order of 100,000 women's lives a year.

When Paul Ehrlich wrote his well-known book The Population Bomb in 1968, Western governments were just beginning to support family planning in countries such as South Korea. At the time, demographers and politicians spoke about "population control," giving the impression that rich countries were telling others how their people should live. Today we know that the surest way to bring down the birth rate is to listen to what people are asking for and to offer them a range of choices. Adults are capable of making up their own minds about what they want.

Many people in the developing world can afford a small payment for modern contraceptives, but poor countries cannot meet the full cost of manufacturing, distributing and promoting them. A few governments, such as those of India and Indonesia, provide contraception free or at subsidized prices. Yet many nations are too impoverished or too corrupt to make family planning a priority. For many of the hundreds of millions of people around the world who live on a dollar a day or less, donations from rich countries are essential--and wanted.

This consensus achieved public prominence in 1994, when the United Nations organized the International Conference on Population and Development in Cairo. The program agreed to at Cairo broadened the traditional scope of population activities to include not only family planning but also efforts to reduce maternal mortality, to treat sexually transmitted infections and to slow the spread of AIDS. The price tag foreseen for the year 2000 was $17 billion, of which $6.5 billion (in 1998 dollars) was to come from developed nations.

Will that money be available? Not on present showing. In 1998 the total flow of foreign aid from rich to poor countries was the lowest in 30 years. Of this amount, only about 3 percent was allocated to assist family planning and reproductive health. Indeed, the U.S. has cut its funding for international family-planning programs over the past few years.

Developed countries last year provided only one third of the money they had pledged to give at Cairo. Because of the shortfall, even meeting the growing cost of contraceptives and of antibiotics to treat sexually transmitted diseases will be difficult in some places.

Many of the parents of the 21st century's children are already born, so credible estimates of the future world population can be made to about 2050. The latest projections from the U.N. Population Division, issued in 1998, envisage a global total between 7.3 billion and 10.7 billion in 2050, with 8.9 billion considered the most likely figure.

It is crucial to realize, however, that this "most likely" number assumes a continuing rise in the rate of use of contraceptives and consequent widespread decline in birth rates. Specifically, it supposes that fertility in developing countries will reach 2.1 by 2050. With current trends, this actually seems unlikely. Large regions of Africa and southern Asia have fertility rates far above 2.1, and unless more funds for family planning become available, I see no reason to think fertility will fall as much as the UN's "most likely" figure assumes.

The 1998 projections necessarily take account of the relentless spread of the AIDS virus in many countries. It now seems probable that well over 50 million people will be infected by 2010--roughly comparable to the number of combatants and civilians killed in World War II. AIDS has lowered average life expectancy by seven years in the 29 most affected African countries. Yet despite this devastating impact, the population of Africa is set to grow from 750 million today to more than 1.7 billion in 2050 because of the momentum built into the population's youth-heavy age structure.

Population projections are not predictions but "what if" statements. If support for family planning remains inadequate, three possibilities, not mutually exclusive, suggest themselves.

First, birth rates could remain higher than the UN assumes they will in its projections. Small variations in the rate at which fertility declines in the next few decades will have profound consequences well into the 22nd century. For example, if Nigeria, now at a population of 114 million, were to achieve a replacement-level fertility of 2.1 in 2010, its population would stabilize at 290 million in about 2100. If the country did not reach 2.1 children for each woman until 2030, the population would rise to 450 million, corresponding to a population density 40 percent greater than that of the Netherlands today. If replacement level fertility does not arrive until 2050, Nigeria's population could theoretically reach 700 million. In fact, disease or starvation would limit population in a most inhumane way long before then.

The second possible outcome of a failure to expand family planning is that some governments might be tempted to impose strict population-control measures such as those adopted by China. In the 1950s and 1960s Mao Tse-tung encouraged large families for ideological reasons. (The Taiwanese, who had excellent access to contraceptives, had one of the quickest fertility declines in history.) By the time the Chinese woke up to the need to slow their growth in 1979, the momentum was so great that the state felt compelled to limit couples to just one child. Even with this policy, the number of Chinese grew from 989 million in 1979 to 1.25 billion today--a gain only slightly less than the total population of the U.S., in a country of roughly the same size.

A third possibility is that abortion rates may rise. Each woman around the world now averages one induced abortion in her lifetime. A recent calculation based on African data suggests that if contraceptives are not available to meet the growing demand, a sixfold jump in abortions will be necessary for birth rates to fall in line with the UN assumptions. That sort of jump would kill thousands of women, because abortions are often performed unsafely.

The success or failure of national family-planning efforts in the opening years of the coming millennium will divide the world along a new geopolitical fault line. Those newly industrialized nations of Asia and Latin America that see family size settle at two or fewer children by about 2010 will join the club of rich Western nations. They will have a slowly aging population, and the number of their citizens older than 60 will double by 2050.

The other set of countries, in Africa and the Indian subcontinent, will be overwhelmed by burgeoning population growth. Vast cohorts of young people will grow up with little education and even fewer job opportunities. Some may form gangs in politically unstable, exploding city slums; others may try to eke out a living by cutting down the remaining forests.

The Cairo conference recognized "the crucial contribution that early stabilization of the world population would make towards the achievement of sustainable development." Transforming the global economy into a biologically sustainable one may well prove the greatest challenge humanity faces. Ultimately, we have to construct a world in which we take no more from the environment than it can replace and put out no more pollution than it can absorb.

If this transition is to succeed, societies will have to reduce both levels of consumption and population sizes. Even today it would be impossible for the planet to sustain a Western standard of living for everyone. Many experts predict that a billion people will be facing severe water shortages by 2025.

Fortunately, much expertise has accumulated about how to make family planning available. The cost to developed countries of meeting this vital need is less than $5 per person per year. That amount is trivial in comparison with the financial, environmental and human costs of inaction.

SEEKING COMMON GROUND: DEMOGRAPHIC GOALS AND INDIVIDUAL CHOICE. Steven Sinding, with John Ross and Allan Rosenfield. Population Reference Bureau, Washington, D.C., May 1994.

HOPES AND REALITIES: CLOSING THE GAP BETWEEN WOMEN'S ASPIRATIONS AND THEIR REPRODUCTIVE EXPERIENCES. Alan Guttmacher Institute, New York, 1995.

SEX AND THE BIRTH RATE: HUMAN BIOLOGY, DEMOGRAPHIC CHANGE, AND ACCESS TO FERTILITY-REGULATION METHODS. Malcolm Potts in Population and Development Review, Vol. 23, No. 1, pages 1-39; March 1997.

6 BILLION: A TIME FOR CHOICES. THE STATE OF WORLD POPULATION 1999. UNFPA, United Nations Population Fund, New York, 1999. Available at www.unfpa.org/ swp/swpmain.htm on the World Wide Web.

LET EVERY CHILD BE WANTED: HOW SOCIAL MARKETING Is REVOLUTIONIZING CONTRACEPTIVE USE AROUND THE WORLD. Philip D. Harvey. Greenwood Publishing, 1999.

PHOTO (COLOR): CONTRACEPTIVES are sent to markets in areas of Bangladesh by boat as part of a campaign originally established by Population Services International. Subsidized condoms and oral contraceptives are sold alongside other goods in shops and kiosks, thus keeping distribution costs low and making the products available to as many people as possible.

GRAPH: CONTRACEPTIVE USE has a marked effect on the average number of children that women have. Fertility rates are conspicuously lower in regions where family-planning assistance is easy to obtain.

GRAPH: FUTURE SIZE OF WORLD POPULATION depends critically on how soon it reaches replacement-level fertility, the point at which each woman bears on average about 2.! children. Projections indicate that faster progress toward lowering fertility could have a large impact.

GRAPH: MANY WOMEN in low-income countries say they would use family-planning services if these were easily available. Some indicate that they would limit the size of their families; others would use the help to space their pregnancies further apart.

GRAPH: USE OF CONTRACEPTION has increased around the world. In addition, more women are employing modem methods, as compared with traditional techniques such as coitus interruptus and abstinence. Yet large disparities between richer and poorer countries persist.

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By Malcolm Potts

MALCOLM POTTS is a British physician who also holds a doctorate in embryology, which he earned at the University of Cambridge. For the past 30 years, he has worked with a variety of groups in the design and implementation of family-planning services and in AIDS prevention. He is a board member of Population Services International, among other organizations, and the author or co-author of several books on aspects of human fertility. Last year Potts published Ever Since Adam and Eve: The Evolution of Human Sexuality. He is Bixby Professor in the School of Public Health at the University of California, Berkeley.