A ban on late-term, or partial-birth, abortions signed into law by US President George W Bush late last year has been ruled unconstitutional.
The judge's ruling is the first federal court decision against the law.
San Francisco-based District Court Judge Phyllis Hamilton said the law was unconstitutional because it was vague and posed an undue burden on abortion rights.
She sided with Planned Parenthood in its lawsuit against the federal government, which had defended the Partial-Birth Abortion Ban Act of 2003.
The law also faces challenges in federal courts in New York and Nebraska.
