Abortion rates in the United States have hit their lowest level since 1974, a new study has found.

The availability of emergency contraception - the so-called "morning after pill" - played a key role in the decline, according to research by the Alan Guttmacher Institute (AGI), a sexual health research organisation.

The rate of abortion among poor women has increased substantially, which is very bad news Steven Sinding, IPPF

The fall in abortion rates could also be explained by more restrictive state laws and lack of access to abortion providers.

The results were published a week before the 30th anniversary of the decision that legalised abortion in the country.

Good news/bad news

The director general of the International Planned Parenthood Federation (IPPF) said the study was "on balance good news".

"A combination of access to emergency contraception and a significant decline in unwanted pregnancies has reduced demand [for abortions], which is very good news," Steven Sinding of IPPF told BBC News Online.

US abortion facts

1.31million abortions in 2000, down from 1.61million in 1990 21.3 abortions per 1,000 women nationwide Number of providers fell from 2,042 in 1996 to 1,819 in 2000

"But the rate of abortion among poor women has increased substantially, which is very bad news," he added.

The US abortion rate rose sharply after legalisation in 1973, but has declined steadily since 1981.

It fell to 21.3 per 1,000 women in 2000, the year on which AGI report is based.

It found that the number of abortion providers has been falling steadily as well, from 2,042 in 1996 to 1,819 in 2000.

Limited access

"One third of American women [of childbearing age] now live in counties with no source of abortion services," AGI found.

A quarter of women having abortions had to travel more than 50 miles, and 8% had to travel more than 100 miles, the study showed.

The District of Columbia - home of Washington, DC - had the highest rate of abortion, at 68.1 per 1,000 women.

Rural Wyoming was at the bottom of the table with one per 1,000 women.

Emergency contraception plays a role

"The United States has one of the highest rates of abortion among industrialised countries outside of eastern Europe, in large part because we have high levels of unintended pregnancy," said Sara Seims, president of AGI, an affiliate of the Planned Parenthood Federation of America.

Abortion is one of the most divisive issues in US politics. It was legalised in 1973 by the Supreme Court's Roe v Wade decision.

The nine-member court ruled that a woman's right to terminate her pregnancy was protected by the right to liberty enshrined in the 14th amendment of the US Constitution.

AGI's findings were based on a survey "of all known abortion providers in the United States".

The institute has been collecting abortion rate data since 1973. Its last comprehensive survey was in 1996.