Image caption Sonograms are usually performed at regular intervals during ordinary pregnancies

A federal judge in Texas has ruled key parts of a new state law which requires women seeking an abortion to undergo a sonogram are unconstitutional.

Judge Sam Sparks upheld the requirement of the law, due to take effect on Thursday, that sonograms be performed.

But he struck down provisions requiring doctors to describe the ultrasound foetus images to patients and that women should hear those descriptions.

He said this would violate free speech rights of both doctors and patients.

The New York-based Center for Reproductive Rights had sued to block the legislation, which was signed in May by Republican Governor Rick Perry, now a White House contender.

'Permanently branding women'

Tuesday's ruling said that requiring doctors to describe a foetus' features would violate the First Amendment of the constitution.

He said this provision "compels physicians to advance an ideological agenda with which they may not agree, regardless of any medical necessity and irrespective of whether the pregnant women wish to listen".

Judge Sparks also struck down a requirement meaning women could only get around the provision if they signed a statement that they became pregnant because of sexual assault or incest.

He wrote that this would require women to disclose "extremely personal, medically irrelevant facts" that will be "memorialised in records that are, at best, semi-private".

"[It] is difficult to avoid the troubling conclusion the Texas legislature either wants to permanently brand women who choose to get abortions, or views these certifications as potential evidence to be used against physicians and women," Judge Sparks wrote.

Supporters have said the law would make certain that women in Texas understood what an abortion entailed.

But opponents have argued the measure could prompt pregnant women to make decisions against their will.

Nancy Northup, head of the Center for Reproductive Rights, told the Associated Press the ruling was a "huge victory for women in Texas and a clear signal to the state legislature that it went too far when it passed this law".

Doctors found breaking the law would face a $10,000 (£6,000) fine and an automatic loss of their medical licences.

Though abortion sonograms are common, they are not always deemed necessary.