Men and women in developing nations are marrying later, having fewer children and having them later in life, U.N. demographers reported on Tuesday.

As a result of these trends, average fertility in poor countries has for the first time fallen below three children per woman, according to the latest data from the U.N. Population Division, which looked at 192 countries for its latest report on population trends.

Fertility in the developing world today averages around 2.9 children per woman, the division reported.

In 20 developing nations, fertility has fallen below 2.1 children per woman, the birth rate generally seen by population experts as replacement-level fertility.

Nearly a quarter of all women aged 25 to 29 years old were single in the 1990s compared to 15 percent in the 1970s, according to the new report. Among men in the same age group, 44 percent were unmarried in the 1990s compared to 32 percent two decades earlier, the report said.

It did not give figures on the ages of first-time parents but said they were having children later than in the past.

In a major shift, U.N. demographers had reported three years ago that fertility rates in much of Asia, Africa and Latin America had unexpectedly begun dropping, easing fears of a future global population explosion that would leave the world overcrowded and short of needed resources.

In a sign the trend was accelerating, the demographers predicted two years ago that fertility in most of the developing world would fall below the replacement level before the end of the 21st century.

In a related trend, use of contraceptives has jumped around the world, with 52 percent of all women either married or in a long-term relationship using some kind of birth control in the 1990s, up from 38 percent in the 1970s.

In developing nations, the percentage of women in some sort of union using contraception rose to 40 percent in the 1990s compared to 27 percent in the 1970s, the report found.

Government policies have played a key role in bringing down fertility rates, the report said. By 2001, 92 percent of the world’s countries supported family planning and distributed contraceptives, it said.

While 75 percent of governments distributed them directly through government facilities, 17 percent distributed them indirectly, through private organizations such as family planning associations, it found.