Women in the developing world now give birth to fewer than four children on average, according to a major United Nations study on fertility.
China has seen one of the sharpest drops in fertility levels
The average number of births has fallen from 5.9 children in the 1970s to 3.9 in the 1990s, it says.
In 20 countries, births have now fallen beneath the number needed to maintain current population levels.
The UN Population Division's World Fertility Report says improved contraception is behind the fall.
"Between 1970 and 2000, the world population experienced a major and unprecedented reduction of fertility levels, driven mostly by a decline in developing countries," it says.
China, which has a strict one-child per family policy, saw the most dramatic drop - with an average of four fewer children per woman over the last 30 years.
Algeria, Iran, Mexico, Thailand, Tunisia and Turkey have also reported significant declines, the report says.
But in 21 sub-Saharan African countries, fertility has declined slowly or not at all.
The 20 countries where fertility is at or below the level needed to sustain the current population include China, Hong Kong, Macao, North Korea, Iran, Kazakhstan, Singapore, Thailand, Armenia, Cyprus, Georgia, Barbados, Cuba, Guadeloupe, Martinique, Puerto Rico, Saint Lucia, Trinidad and Tobago and Chile.
Those countries recording large falls in birth rates have governments that have decided to promote the use of contraceptives to modify fertility levels.
"A tremendous increase has taken place in the use of family planning," the report says, adding that over half of all women in the world who are married or in some form of union now use contraceptives, compared with 38% during the 1970s.
The developing world has seen a particularly sharp increase - from 27% using contraceptives in the 1970s to 40% by the 1990s.
In both the developed and developing world, women are increasingly choosing to get married later and to postpone having children.
The average age for marriage for women is now 23, and 27 for men.