 The
average number of births has fallen from 5.9 children in the 1970s to 3.9
in the 1990s, it says. Planned families 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. 
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.
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