The Oklahoma Supreme Court has ruled that a proposal to give "personhood" rights to human embryos is unconstitutional.
The ruling comes as supporters of the measure are trying to gather signatures to put it before Oklahoma voters on the November ballot.
Their effort would amend the Oklahoma Constitution to define a fertilised human egg as a person.
Opponents contend it would ban abortions without exception.
They also say it would interfere with a woman's right to use certain forms of contraception and some medical procedures, such as in vitro fertilisation.
The state's highest court ruled unanimously that the proposal violates a 1992 US Supreme Court decision and "is clearly unconstitutional".