The California Supreme Court ruled yesterday that both members of a lesbian couple who plan for and raise a child born to either of them should be considered the child's mothers even after their relationship ends.

The court, stepping into largely uncharted legal territory concerning same-sex couples and parenting, issued decisions in three cases, ruling that women whose partners gave birth had parental rights or obligations in all three.

The cases involved a request for child support, a petition to establish parental rights and an attack on a lower court ruling issued before a child's birth that the child should have two women listed as parents on her birth certificate.

"We perceive no reason," the Supreme Court ruled, "why both parents of a child cannot be women."

Courts in about half the states have allowed members of same-sex couples to adopt their partners' children. Yesterday's decisions considered the separate question of whether the law could require former members of such couples to assume parental rights and obligations.

The cases all involved unorthodox conceptions, and the Supreme Court struggled to apply sometimes inapt state laws to them.

"While scientific advances in reproductive technology now afford individuals previously unimagined opportunities to become parents," wrote a dissenting justice, Kathryn M. Werdegar, "the same advances have also created novel, sometimes heartbreaking issues concerning the identification of the resulting children's legal parents."

The decisions broke new ground, advocates on both sides agreed.

"It is unprecedented around the country," said Joan Hollinger, who teaches adoption law at the University of California, Berkeley, "to have a state's highest court recognize that in the absence of an adoption, and even in the absence in some instances of a domestic partnership agreement, that two men or two women could be the full legal parents of a child born through assisted reproduction." Ms. Hollinger submitted supporting briefs on behalf of the children involved.

But the rulings troubled lawyers for groups defending what they called traditional values.

"You've essentially begun to undermine and unravel the family," said Mathew D. Staver of Liberty Counsel, a law firm that submitted briefs arguing against the recognition of two same-sex parents.

The decisions may also have implications for same-sex marriage in California. The question of whether the state Constitution requires the recognition of such marriages is before a state appeals court.

"If these cases are any indication," Mr. Staver said, "it makes it look like they're tending toward recognition of gay marriage."

The only one of the three decisions that provoked dissents, and the only one that seemed to leave open the possibility of an appeal to the United States Supreme Court, involved a woman identified as K. M., who provided an egg to her lesbian partner. The partner, E. G., gave birth to twin girls. K. M. had signed a form giving up her claims to any child at the time of the donation but, after the couple broke up, filed a lawsuit to establish her parental rights.

The Supreme Court, in a 4-to-2 decision, ruled for K. M. notwithstanding a state law that says a man who donates his semen to impregnate a woman who is not his wife is not a legal father. Justice Werdegar, dissenting, suggested that treating the donation of sperm differently from the donation of an egg "inappropriately confers rights and imposes disabilities on persons because of their sexual orientation" and so "may well violate equal protection."

The United States Supreme Court does not ordinarily hear cases considering questions of pure state law. But it could hear a claim under the federal Constitution's equal protection clause.

Diana Richmond, who represented E. G., said her client had not decided whether to appeal.

K. M., who declined to give her full name "to protect the privacy of my children," said in an interview arranged through her lawyer that she welcomed the decision.

"Next to the day my daughters were born," she said, "this is the happiest day of my life."

The other two decisions yesterday did not involve donated eggs.

In one, the court ruled that a woman identified as Elisa B. must pay child support to her former partner, who gave birth to twins while the women were a couple. The court made its determination based on commitments the women had made to each other and their treatment of the children each of them had while they were together.

In the third decision, the court ruled that a woman identified as Kristine H. did not have the right to challenge an earlier decision that granted her former lesbian partner parental rights, including putting her name on the birth certificate of Kristine H.'s child in the space provided for "father." The court based its decision on Kristine H.'s participation in the earlier proceeding.

Lawyers on the losing sides of the decisions said that the rulings would give rise to confusion between competing state laws, with someone said to be parent in California, for instance, not considered one if she moved to Texas.