The ruling by federal Judge William Pauley blocks a law that was supposed to go into effect Thursday. It would require pregnancy centers to disclose whether they have licensed medical staff and what they do to protect client privacy.
Abortion rights advocates say the centers try to deceive women by opening near offices that offer abortions and implying they will give referrals to women considering ending their pregnancies.
The judge agreed with several centers that are suing that the law, supported by abortion rights groups, appears to be unconstitutionally vague.
Because the law "relates to the provision of emergency contraception and abortion - among the most controversial issues in our public discourse - the risk of discriminatory enforcement is high," the judge said.
The city could not confirm that any centers had been prosecuted under anti-fraud statutes, Pauley noted.
"The ordinance threatens non-medical, pro-life pregnancy services centers with heavy fines and possible closure if they don't provide posted, printed, and oral notices crafted by the city that encourage women to go elsewhere," said the Alliance Defense Fund, a conservative religious rights organization that sued on behalf of two pregnancy centers and a maternity home.