In 1973 the pivotal United States Supreme Court decision Roe v Wade legalised abortion across the country and became a touchstone for 50 years of conflict between the pro-life and pro-choice movements.

A leaked draft of a new Supreme Court ruling shows Roe v Wade is on the cusp of being overturned, intensifying furious debate in the US and potentially imperilling a right that millions American women have had for decades.

But how did this decades-old ruling come to hold such significance in the global discourse about women’s rights, and what could overturning it mean?

What was the decision in Roe v Wade?

In 1970, Norma McCorvey was a Texas mother of two children who became pregnant with a third. She wanted an abortion but the state’s laws denied access to the procedure unless the mother’s life was in danger, so McCorvey launched a legal challenge. She was given the pseudonym “Jane Roe” to protect her identity but later went public. Henry Wade, the opposing Texas attorney-general, also gave his name to the case.

The case did not come out of nowhere. As the feminist movement grew and the sexual revolution swept America from the 1960s, pressure grew from women's groups to challenge abortion restrictions.

After a series of decisions and appeals, McCorvey’s case reached the United States Supreme Court, the highest court in the country. The court ruled by a 7-2 majority in 1973 that the Texas law was unconstitutional. By then McCorvey had given birth and put the child up for adoption.

How did they decide that?

The United States Constitution is the country's founding document. It makes no mention of abortion.

Nonetheless, the justices of the Supreme Court reasoned that a right to privacy, which the court had previously found was implicit in several parts of the constitution and covered things such as contraception and procreation, extended to a right to have an abortion. In the majority opinion, Justice Harry Blackmun wrote that the costs of denying access to abortion were high, including physical injuries during pregnancy, financial and psychological costs of raising an unwanted child and social stigma.

What was the effect of the decision?

States such as Texas were no longer allowed to have laws tantamount to banning abortion, a radical step that followed decades in which many states had criminalised the procedure.

The decision also served as a lightning rod for religious conservatives who argued that life began at the point of conception and that abortion was tantamount to murder.

Even Justice Ginsburg, the late liberal feminist Supreme Court judge, argued the court could have avoided some of that backlash had it decided Roe v Wade on equality rather than privacy grounds, as it would be easier to explain to critics.

Does that mean abortion is available everywhere in the United States?

No. The qualifications Blackmun put on his reasoning have been used to decrease access to abortion over the decades in many states.

Federal and state laws at different times have banned federal medical funding going to abortion providers, banned doctors raising abortion as an option with pregnant women, required doctors delivering abortion services to be tied to local hospitals and restricted abortion to the early stages of a pregnancy.

So what has the Supreme Court done now, and why?

A draft opinion published by the news site Politico indicates that a majority of the Supreme Court has voted to overturn Roe v Wade.

The draft opinion is from a case called Dobbs v Jackson Women’s Health Organisation, which is a challenge by Mississippi’s only abortion clinic to a 2018 state law banning abortions after 15 weeks. That law has an exception for severe fetal defects and medical emergencies but none for pregnancies that are the result of rape or incest. The clinic argues it violates Roe v Wade and later decisions upholding it; the respondents argue that Roe should be reversed.

The draft appears to be a decision written by the conservative Justice Samuel Alito for five judges, a majority, overturning Roe. “Roe was egregiously wrong from the start,” Alito reportedly writes in the document. His view is that the constitution does not actually grant the right to abortion.

Final court decisions can change and even flip from drafts but the result aligns with the composition of the court. There are six conservative judges and three liberals on the court.

Politico claims it obtained the document from “a person familiar with the court’s proceedings in the Mississippi case” along with other materials establishing its legitimacy. Legal experts familiar with the court said it read like Alito’s writing. The court has declined to comment.

A full draft opinion has never leaked before a decision is published in the modern history of the court but there have been numerous leaks of court business.

In 1973, for example, the result of the original Roe v Wade decision was leaked to Time magazine. It had been disclosed by a court clerk with the intention that it not be publicised before the decision, but a scheduling error meant the article was released first.

The court will release its final decision by the end of this term, which will conclude in June or July.

What would happen if the draft ruling becomes final?

If the court repeals Roe, the consequences would be immediate. The Guttmacher Institute, an abortion rights advocacy group, says 22 states have laws that would restrict abortion if Roe were struck down. A further four states - Florida, Indiana, Montana and Nebraska - are also likely to ban or restriction abortion.

Some, such as North Dakota and Mississippi, have “trigger laws” that would come into effect if the Supreme Court repeals Roe. In Mississippi, abortions would be banned unless there was a pending rape trial or the mother’s life was in danger. These trigger laws are a way of showing a state government is doing all it can to ban abortion. But in 13 other states, as well as the District of Columbia, abortion access would be unchanged under state laws that protect it.

Research from scholars at Middlebury College in Vermont and abortion rights groups suggest that ending Roe would have the biggest impact on poorer women who would find it harder to afford the travel, accommodation, time off work and medical expense of an interstate abortion. State-level politics means those in the midwest and south would have their access restricted the most.

Congress could pass legislation to reinstate abortion rights nationwide (or restrict them further if Republicans take control) if the court does overturn Roe, but that is unlikely. In February 2022, the US Senate voted down a bill 46 to 48 that would have granted a federal right to abortion. Sixty votes from either party are required to overcome a filibuster, which is a tactic that can stall a bill.

Why has the court tipped on Roe v Wade?

Abortion is a white-hot political issue in the United States, so presidents have a strong incentive to nominate justices to the Supreme Court who share their view of the subject. In recent years Republicans have been more successful at that.

Republicans took a Supreme Court seat from the Democrats when then-Senate Leader Mitch McConnell refused to hold hearings for President Barack Obama’s pick, the centrist jurist Merrick Garland, in 2016. He had been selected to replace the conservative judge Antonin Scalia.

Former president Donald Trump then courted the evangelical vote despite his three marriages and history of alleged sexual assaults and harassment by appointing Supreme Court judges sceptical of Roe.

“This is the most openly pro-life judicial nominee to the Supreme Court in my lifetime,” firebrand Republican Senator Josh Hawley said of one of Trump’s three picks, conservative Catholic judge Amy Coney Barrett.

What does this mean for Australia?

It has no direct impact. Abortion is legal in all Australian states and territories but with varying restrictions, including some on abortions after about halfway through a standard pregnancy and a requirement for doctor’s permission. New South Wales was the last state to decriminalise abortion following a long and vicious political fight in 2019.

In each state, politicians rather than judges have been at the forefront of recent debates on abortion because Australia's constitution guarantees far fewer rights than America's. As a result, judges here have much less power to overrule legislators and strike down laws to create new freedoms.

Note: This article has been extensively updated to reflect a report that the Supreme Court is set to overturn Roe v Wade.