A law banning abortion from as early as six weeks into pregnancy has come into effect in the US state of Texas.
It bans abortions after the detection of what anti-abortion campaigners call a foetal heartbeat, something medical authorities say is misleading.
The law, one of the most restrictive in the country, took effect after the Supreme Court did not respond to an emergency appeal by abortion providers.
Doctors and women's rights groups have heavily criticised the law.
It gives any individual the right to sue doctors who perform an abortion past the six-week point.
The so-called "Heartbeat Act" was signed into law by Texas Governor Greg Abbott in May.
But rights groups, including Planned Parenthood and the American Civil Liberties Union (ACLU), then requested that the Supreme Court block the legislation.
In the early hours of Wednesday, the ACLU confirmed that the court had "not responded to our request", adding: "Access to almost all abortion has just been cut off for millions of people."
The group, which says that up to 90% of abortions in Texas take place after six weeks of pregnancy, described the development as "blatantly unconstitutional".
The US women's health group Planned Parenthood also condemned the ban, tweeting: "No matter what, we aren't backing down and we are still fighting. Everyone deserves access to abortion."
In a statement, President Joe Biden denounced the bill as "extreme", warning it would "significantly impair" access to abortion care, particularly for low-income Texans and racial minorities.
The Supreme Court still has the power to overturn the ban at a later stage.
What does the law say?
Since the 1973 Supreme Court decision in Roe v Wade, US women have had the right to an abortion until a foetus is viable - that is, able to survive outside the womb. This is usually between 22 and 24 weeks into a pregnancy.
The so-called Texas Heartbeat Act prohibits abortions after six weeks of a pregnancy - at a point when many women do not know they are pregnant. The American College of Obstetricians and Gynaecologists has said the term "heartbeat" is misleading, and that what is being detected at this stage is "a portion of the foetal tissue that will become the heart as the embryo develops".
The Texas law enforces its ban with an uncommon approach: it empowers any private citizen to sue anyone who "aids and abets" an illegal abortion.
The legislation makes an exception in the case of medical emergency, which requires written proof from a doctor, but not for pregnancies resulting from rape or incest.
Texan women who wish to have an abortion after six weeks will need to travel across state lines, or - as estimated by the pro-choice Guttmacher Institute - an average of 248 miles (399km).
How is this different from other restrictions?
Most abortion restrictions that have been proposed before have relied on criminal penalties or some form of regulatory punishment.
The Texas law instead authorises "a private civil right of action", which allows people to sue to enforce the law even if they themselves have not been harmed.
An ordinary American, from Texas or elsewhere, may now be able to seek up to $10,000 (£7,200) in damages in a civil court against abortion providers and doctors - and possibly anyone at all involved in the process. That means people like clinic staff, family members, or clergy who encourage or support the procedure could, in theory, be sued.
Turning over enforcement of the Heartbeat Act to private citizens instead of government officials likely means that - in the absence of Supreme Court intervention - the law cannot be challenged until a private citizen seeks damages.
Kim Schwartz of the Texas Right to Life organisation, which supports the measure, told the BBC most anti-abortion laws are "held up in the court system for years" and this "thwarts the will of the people". She argued that courts would require "a credible claim that an illegal abortion occurred" and would still undergo fact-finding processes.
But the ACLU and other critics have suggested the Texas law will champion "a bounty hunting scheme" of costly "vigilante lawsuits" designed to harass women seeking an abortion. The ACLU noted tip lines have already been set up by anti-abortion groups.
How do Americans feel about abortion rights?
Abortion has long been one of the country's most contentious social issues. However, polls from the Pew Research Center indicate nearly six in 10 Americans believe abortion should be legal in all or most cases.
This number has remained relatively stable over the past two decades, but masks a partisan divide: only 35% of Republicans support that position.
In conservative Texas, an April poll found nearly half of the state's voters support a six-week ban on abortions.
What's the background?
The Texas law is one of many recent efforts to limit abortions in states controlled by Republicans.
Three others - Idaho, Oklahoma and South Carolina - have passed six-week ban bills this year, all of which have been stalled by legal challenges and are yet to go into effect.
According to the Guttmacher Institute, 2021 has already seen more restrictions enacted in any year since the Roe v Wade decision nearly 50 years ago.
Supporters of these measures have been emboldened by the makeup of the Supreme Court, which currently has a 6-3 conservative lean.
Pro-choice Americans have expressed fear that, if given the chance, a majority of the high court's jurists will opt to overturn national abortion rights. This would not make abortion illegal across the country, but rather, would return that decision to each of the states.
Some Democrat-led states, like New York, have enacted measures to safeguard abortion access in the event of this scenario.