Oklahoma legislators have passed a law banning abortion after conception, which critics say is the most restrictive such measure in the US.
The Republican-led bill would prohibit all abortions, except to save the life of the woman or if the pregnancy is the result of rape or incest.
It comes after a leak suggested the US Supreme Court may quash the 1973 ruling that legalised abortion nationwide.
Republican-led US states are ramping up bills to limit abortion access.
On the other side, US Senate Democrats last week tried to pass a bill that critics said would allow abortion through all nine months of pregnancy. It was stopped by Republicans.
Opinion polls find most Americans are in favour of abortion access, though surveys also show public support for allowing the procedure drops sharply after the first trimester of pregnancy.
Abortion is generally legal in US states that do not restrict it up to about 24 weeks of pregnancy, which is near the end of the second trimester.
Oklahoma has recently passed several laws aimed at setting it apart as the most “pro-life” state in the US.
The bill, which passed overwhelmingly on Thursday, is modelled on a Texas anti-abortion law that allows anyone to sue abortion providers.
But the Oklahoma bill bans abortion even earlier than Texas, where it is not legal after six weeks. This is around the time cardiac activity can be detected in the embryo, and before many women realise they are pregnant.
The Oklahoma measure defines fertilisation as the “fusion of a human spermatozoon with a human ovum”. Banning abortion after conception has been a long-held goal of national anti-abortion groups.
The bill still needs to be signed into law by the state’s Republican governor, who has vowed to approve any anti-abortion legislation that comes to his desk.
The Oklahoma measure allows exemptions in cases of rape and incest, but only if reported to the police. It does not ban contraception or morning-after pills.
In the past three months, Oklahoma has passed two other anti-abortion bills, which have already caused most abortion providers in the state to close.
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By early July, a majority of conservative justices on the US Supreme Court could overturn the Roe v Wade decision that legalised abortion nearly half a century ago, according to a recently leaked draft opinion.
Such a decision would not ban abortion throughout the US, but allow state legislatures to determine whether to restrict abortion access, or ban it altogether.
Oklahoma is one of nearly half of US states with so-called trigger laws, which would immediately outlaw abortion access if Roe v Wade were overturned.
