LONDON (AP) - A British judge ruled Tuesday that more evidence is needed before deciding whether to grant a mother's wish to have her mentally disabled daughter sterilized - a case that is troubling medical ethicists.

The 21-year-old woman, who has a significant learning disability and has been identified only as P, already has one child who is being cared for by her mother, Mrs. P. The woman is scheduled to give birth via cesarean section on Wednesday and her mother had proposed that doctors could sterilize her daughter at the same time.

"From my point of view, I want the best for my daughter," Mrs. P told the court. "Obviously, we can't carry on supporting more and more children."

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She said if her daughter had more children in the future, they would have to be cared for by the state.

The decision Tuesday means no sterilization will take place this week.

The daughter was represented in court by a government-appointed lawyer. It is unclear whether the daughter objects to having her tubes tied. The Court of Protection, where the case is being heard, would not comment on ongoing legal cases.

Because her daughter is mentally disabled, the matter has to go through the courtsThe mother is asking the court to decide whether her daughter is able to make her own decisions about contraception. If the court agrees she lacks that capability, the mother asked that doctors be given permission to sterilize her.

The presiding judge described P as a "sexually healthy and active" young woman and said it was hardly surprising there was serious concern about future pregnancies.

But some critics questioned whether the radical step was necessary and worried about the precedent it would set.

"This is eugenics if they are doing this because she's mentally disabled," said George Annas, chair of the department of health law and bioethics at Boston University. "This decision needs to be made based on the person's best interests, not the best interests of society or her caregivers."

Annas said to justify sterilization of a disabled woman who doesn't have the capacity to consent, doctors needed to prove why a less invasive method like birth control wouldn't work.

Under Britain's Mental Capacity Act, passed in 2005, courts can decide on medical treatments for people deemed incompetent. Most cases, however, involve decisions about withdrawing intravenous food from patients in a permanent vegetative state.

British courts can also order abortions for women who lack the mental capacity to consent or compel lifesaving medical treatments that mentally disabled people might otherwise refuse.

"It is a drastic step to sterilize someone against their will simply because they lack mental capacity," said Mark Goldring, chief executive of Mencap, a British charity for people with learning disabilities. "It is a gross invasion of someone's basic rights unless there are clear medical grounds ... we urge the court to tread very carefully."

Other experts said sterilizing a woman who is mentally incompetent could be warranted in exceptional cases, such as if the woman isn't able to take care of a baby.

"The issue is also whether or not you're being kind or being cruel by repeatedly exposing her to the dangers of pregnancy and the risk of a C-section without having the opportunity to keep the child," said John Harris, a professor of bioethics at the University of Manchester. "If the alternative is to expose her to further pregnancies only to see her children repeatedly taken away, sterilization could be the lesser of those evils."

Harris said people did not have an inherent right to have as many children as they like, especially if that imposed huge costs on society.

The case is now scheduled to be heard in April after the court requested more expert evidence.