At current fertility rates in most developing countries, women give birth to an average of more than three children over their lifetimes. In some countries women average five or six children, or even seven children in a few countries. These numbers, of course, far exceed "replacement-level" fertility of about two children per woman—the fertility rate at which population growth would level off and population size would eventually stabilize. Even if the next generation has fewer children than the previous one, the population will grow as long as couples average more than two children each.

Because of high fertility in the past, most developing country populations are young. In the developing world as a whole, half of the population is under age 23 (122). In sub-Saharan Africa children under age 15 comprise almost 40% of the population (2). As young people reach childbearing age themselves, most will have several children of their own. Even after fertility falls to replacement level, it takes at least another generation or two for population size to stabilize. Also, it takes at least 15 years for the smaller size of age groups to have an appreciable effect on food demand and resource needs (33).

Time is of the essence. Each 20-year delay in reaching replacement-level fertility of about two children per woman would add at least 1 billion people to the world's eventual stable population size (33). If replacement-level fertility were reached worldwide by the end of this decade—which is virtually impossible—world population would eventually level off at less than 9 billion. If, at the other extreme, replacement level were not reached until the year 2080—which might be the case if family planning programs did not expand to meet the needs of larger populations and the rising interest in contraception—the world's ultimate population size would be at least 14 billion (33, 36). The actual date—and thus the ultimate world population size—is likely to be somewhere in between.

If fertility were to decline to replacement level by the year 2050, the world population would be about 9.4 billion, according to the UN "medium" projection, the case considered most likely to occur. In this case world population would level off at 11.6 billion around the year 2100 (3, 36). To reach replacement-level fertility by 2050 would require that contraceptive prevalence—the percentage of married women of reproductive age using contraception—rise from the 1990 level of about 50% in developing countries to 73% by 2025, matching current levels in developed countries. The reproductive revolution is most likely to be sustained if commitment to family planning programs in developing countries expands to meet people's increasing interest in having smaller families and the large amount of unmet need for family planning (78, 79).