Workers at Preterm Inc., a Brookline abortion clinic to which the University Health Services sometimes refers students, voted Wednesday to strike after management refused to accept union demands for a closed shop.

Elliot Small, president of Hospital and Health Care Workers Local 1199, said yesterday Preterm was not bargaining in good faith and was seeking to avoid a contract with the union altogether.

There has been no contract agreement since workers voted to establish a union in May 1975.

Diane Richards, director of Preterm, said yesterday she was opposed to the union-shop clause because it would deny freedom of choice to prospective employees who don't want to join the union.

"It is inconsistent with our philosophy which is based on full freedom of choice for every individual--employees as well as patients," she said.

However, Small said yesterday Preterm was primarily against the union for economic reasons and because a contract would reduce Preterm's control over its workforce.

Small said yesterday that if there is no progress before October 16--the day the strike vote becomes effective--the union, representing 68 workers at Preterm, would file charges of unfair labor practices with the National Labor Relations Board.

Richards said yesterday Preterm plans to remain open in spite of the strike. "I anticipate that we will do everything we can to hire permanent replacements" for strikers, she added.

But, Small said such an action would be illegal. "Management threats to hire replacements before a strike begins are illegal attempts to intimidate workers in an unfair labor practices strike," Small said, adding he expects the strike to close the clinic.

Harvard Unaffected

University Health Services officials said yesterday the possible closing of the clinic, which performs 800 abortions a month in addition to providing other gynecological services, would not significantly affect the Harvard community.

Dr. Warren Wacker, director of the Health Services, said abortion referrals by UHS averaged less than one a week. Most of the referrals, he added, were to the Crittenton-Hastings Clinic in Brighton.