LOUISVILLE, Ky. -- Abortion-rights defenders have expanded their legal fight against Kentucky to try to block the state's most restrictive abortion measure, which would mostly ban the procedure once a fetal heartbeat is detected.

The American Civil Liberties amended a federal lawsuit Friday to take aim at the so-called fetal heartbeat bill. The measure won final passage late Thursday in Kentucky's Republican-dominated legislature and was sent to the state's anti-abortion governor, Republican Matt Bevin.

Bevin's office did not immediately comment on the latest legal action.

ACLU attorneys say the heartbeat bill would prohibit 90 per cent of abortions in Kentucky.

A fetal heartbeat can be detected as early as six weeks into pregnancy, before many women know they're pregnant.