A brewing Bay Area court battle is delving into the largely uncharted area of lesbian partners' parental rights, a case legal observers say could be precedent-setting and help clarify the rights of sperm and egg donors when they involve domestic partners.

At a time when gay rights have moved to the forefront of national debate, the case set to be heard next month by a San Francisco appellate court has an added wrinkle:

Both women, who were in a committed relationship for 10 years, are biologically related to their fraternal twin daughters. One provided the eggs; the other gave birth to the girls eight years ago.

A superior court commissioner in Marin County ruled in January that the egg donor, by signing a routine consent form at a fertility clinic, promised that she would have no parental rights. Commissioner Randolph Heubach awarded sole parental rights to the donor's partner, the birth mother.

The National Center for Lesbian Rights in San Francisco filed a brief with the 1st District Court of Appeal in support of the egg donor, as part of its fight for equal rights for gay and lesbian parents.

``We want the same legal standard and rules applied to our families as any other. Were the two women a heterosexual married couple, never for one moment would they be in court with the genetic mother fighting to maintain her place in her children's lives,'' said Kate Kendell, the center's executive director. ``The fact that she lived in the home and reared the girls trumps the contract that she might have signed.''

In an interview, the egg donor, who is identified only by her initials K.M. in court documents, ticked off a series of reasons about why she was not an average anonymous donor.

``We were registered domestic partners. We lived together. We bought a home together. We were on a family health plan together. We tried to get pregnant for over a year. We were given a baby shower. We always knew we would be parenting together,'' she said. ``Never, ever, ever would I have dreamed that something like this was possible.''

K.M.'s former partner, identified as E.G., would not speak to the Mercury News but released a statement through her attorney, Diana Richmond of San Francisco, saying that her former partner had a chance to tell her story before a judge and he ruled against her.

``I was trying to become a parent before I ever met K.M.,'' according to the statement. ``I made it clear to her from the beginning that I wanted to be a single parent, and that I would accept her ovum donation only if it was truly a donation, and I would be the sole legal parent.''

After trying unsuccessfully for more than a year to become pregnant, K.M. and E.G. met with fertility specialists about their options in 1994. Doctors at the University of California-San Francisco's Center for Reproductive Health Research and Policy told K.M. that her uterus was damaged and she could not carry a baby to term. E.G., on the other hand, had a healthy uterus, but her eggs were no good.

K.M. said that although it was unconventional -- especially at the time -- doctors at the clinic suggested she provide the eggs and, with an anonymous sperm donation, they could be fertilized and implanted in her partner's uterus.

``We looked at each other and we thought, `Wow, we never even knew that was possible,' '' said K.M., who spoke to the Mercury News on the condition her name not be used. To protect the privacy of the children, family court documents don't include the full names of the women.

The girls, now 8, were born in December 1995. They were premature and spent weeks in the hospital, with both women at their sides. One afternoon, K.M. and E.G. stole away and bought wedding rings to reaffirm their commitment to each other. They exchanged them on Christmas Day.

At the time, in vitro fertilization was not as common among lesbian partners as it is today, so the document that K.M. signed treated her as an anonymous donor, even though she argues that neither she nor her partner intended it that way. The court in Marin ruled the document spoke for itself.

``By voluntarily signing the ovum donation form, K.M. was donating genetic material,'' Heubach wrote in his decision. ``Her position was analogous to that of a sperm donor, who is treated as a legal stranger to a child if he donates sperm. . . . The Court finds no reason to treat ovum donors as having greater claims to parentage than sperm donors.''

Three years after their children were born, K.M. said, her former partner dropped a bombshell during an argument about how and when to tell the children about their unusual biological background: E.G. said she didn't want to tell the children, and reminded K.M. that she had signed a form agreeing to relinquish all rights.

They continued to live together as a family until the girls were 6 1/2. In 2002, the couple separated, and E.G. moved to Massachusetts with the children. Since then, K.M. said, her phone calls and visits have been restricted.

Richmond said her client is under no obligation to grant visitation to K.M., but has let her see the girls because they have bonded with her.

``The choices in this case were made at the outset, and much as K.M. might regret it afterward, it's been decided in a logical way,'' Richmond said.

K.M.'s attorney, Jill Hersh of San Francisco, disagrees.

``It doesn't matter how many papers were signed; it's unconscionable to me that these girls are being deprived of one of their parents,'' Hersh said. ``Our children's rights in the families of today have to be protected. It's time for the law to catch up.''

Infertility specialists say they try to do the right thing, but unconventional families such as E.G. and K.M.'s pose new challenges.

Dr. David Adamson, director of Fertility Physicians of Northern California in San Jose and Palo Alto, said he insists that all unmarried couples -- whether gay or not -- write up a legal document outlining each person's parental rights and responsibilities.

If they refuse, he won't treat them.

``By strict interpretation, she signed her rights away. But I would think after eight years of parenting, she's earned some rights,'' Adamson said.