Abortions were made legal in the United States in a landmark 1973 Supreme Court judgement, often referred to as the Roe v Wade case.
By a vote of seven to two, the court justices ruled that governments lacked the power to prohibit abortions.
Abortion has been a lightning-rod for opinion in the US for decades
The court's judgement was based on the decision that a woman's right to terminate her pregnancy came under the freedom of personal choice in family matters as protected by the 14th Amendment of the US Constitution.
The decision - on 22 January 1973 - remains one of the most controversial ever made by the Supreme Court.
The ruling came after a 25-year-old single woman, Norma McCorvey under the pseudonym "Jane Roe", challenged the criminal abortion laws in Texas that forbade abortion as unconstitutional except in cases where the mother's life was in danger.
Henry Wade was the Texas attorney general who defended the anti-abortion law.
Ms McCorvey first filed the case in 1969. She was pregnant with her third child and claimed that she had been raped. But the case was rejected and she was forced to give birth.
However, in 1973 her appeal made it to the US Supreme Court where she was represented by Sarah Weddington, a Dallas attorney.
Her case was heard on the same day as that of a 20-year-old Georgia woman, Sandra Bensing. They argued that the abortion laws in Texas and Georgia ran counter to the US Constitution by infringing women's right to privacy. They won their case.
The case created the "trimester" system that:
- gives American women an absolute right to an abortion in the first three months of pregnancy
- allows some government regulation in the second trimester of pregnancy
- declares that states may restrict or ban abortions in the last trimester as the foetus nears the point where it could live outside the womb; in this trimester a woman can obtain an abortion despite any legal ban only if doctors certify it is necessary to save her life or health.
But in the following decades anti-abortion campaigners regained some lost ground. More than 30 states have adopted laws limiting abortion rights.
In 1980 the US Supreme Court upheld a law that banned the use of federal funds for abortion except when necessary to save a woman's life.
Then in 1989 it approved more restrictions, including allowing states to prohibit abortions at state clinics or by state employees.
The issue still raises passions and marches by both sides
In 2003, Congress introduced the first major limits on abortion in the US for 30 years when it passed a law banning a particular form of late-term abortion labelled "partial birth" by its opponents.
Legal challenges to the ban, signed into law by President Bush, continue.
The result of these restrictions is that many women have to travel further to get an abortion, often across state borders, and pay more for them. According to the pro-choice movement, poor women are penalised most by these restrictions.
Norma McCorvey announced in 1987 that her rape testimony in 1969 had been false.
Now a born-again Christian, she converted to the pro-life lobby, and two years later Sandra Bensing followed suit.
Norma McCorvey, the case's Jane Roe, is now against abortion
But Ms McCorvey's attorney, Sarah Weddington, insists that the rape testimony was not a factor in the Roe verdict, and that her decision to change sides has no bearing on the ruling.
The greatest court triumph of the pro-life lobby was the Supreme Court's ruling in Planned Parenthood v Casey in 1992.
While upholding the Roe v Wade ruling, it also established that states can restrict abortions even in the first trimester for non-medical reasons.
The new laws must not place an "undue burden" on women seeking abortion services. However, it is the woman and not the authorities who have to prove that the regulations are damaging.
As a result many states now have restrictions in place such as requirements that young pregnant women involve their parents or a judge in their abortion decision.
Others have introduced waiting periods between the time a woman first visits an abortion clinic and the actual procedure.
Some states also provide information that has to be presented to women having abortions that could discourage them from going ahead.