BOSTON — The governor's office has instructed hospitals to cross out the word "father" on birth certificates for children of same-sex parents and substitute "second parent," angering municipal clerks.

Eric Fehrnstrom, a spokesman for Gov. Mitt Romney, said the certificates are legal.

But municipal clerks, who register and store birth records, said the cross-outs could leave the documents open to challenges by passport agents, foreign governments and other officials.

"They should not have a birth certificate that has crosses on it," said Barnstable Town Clerk Linda Hutchenrider, a past president of the Massachusetts Town Clerks Association. "They should be allowed to have a birth certificate that really looks valid."

So far, only lesbian couples have been affected, so the issue of "mother" hasn't come up, The Boston Globe reported.

Hutchenrider sent a letter to Romney in October, asking him to formally revise the birth certificates for children of same-sex couples.

In recent weeks, she discussed the issue with Department of Public Health officials, but Hutchenrider said she was told the forms cannot be changed without the governor's approval.

Fehrnstrom said the health department has been advising hospitals to alter the documents since last year and the governor thinks the hand-altered certificates are valid.

"As long as they're recorded, they're valid," he said.

Fehrnstrom also said the Legislature must authorize any changes to the birth certificates.

In 2004, when Massachusetts became the first state to legalize same-sex marriage, there were 61 children born to married same-sex couples out of about 80,000 children born in the state, according to data from the Department of Public Health. This year, the number was 75 by the end of June.

"It doesn't matter if there's only one or 500," Hutchenrider said. "They all deserve to have proper birth records."