Doctors are reporting two advances that may give women with cancer safer ways to preserve their ability to have children without compromising their chances of beating the disease.

One involves a new way to help women store up eggs before having cancer treatments that often leave them infertile.

The other is a very sensitive method for checking frozen ovarian tissue for abnormal cells that could seed a relapse of cancer if transplanted back into a woman who finished treatment and wanted to have a child.

In a medical first, a Belgian woman recently gave birth after an ovarian-tissue transplant. But doctors have long worried that such tissue might harbor microscopic disease. And, in fact, the new research found signs of cancer in ovarian tissue that two cancer patients had hoped to freeze.

Both studies were presented Monday at a meeting of the American Society for Reproductive Medicine, which issued new guidelines saying that freezing eggs and ovarian tissue are promising but experimental options that should be offered only to cancer patients through programs that ensure they understand the risks.

Neither technique “should be marketed or offered as a means to defer reproductive aging” in healthy women, the guidelines say. At least one business is offering to freeze eggs as for $15,000 as a biological insurance policy for women worried about wanting a baby after they are too old to have one.

Preserving fertility has become a big issue as more young women survive cancer. About 50,000 women under 40 are diagnosed with cancer each year in the United States, and many get radiation or chemotherapy that damages their ovaries.

In a survey reported Friday in the Journal of Clinical Oncology, three out of four breast cancer patients asked their doctors how treatment would affect their fertility, and one in three reported that it affected the kind of treatment they chose.

Some women want to bank eggs or embryos before their ovaries are damaged, but hormones that spur ovulation are not considered safe for them because they raise estrogen, which makes many tumors grow. Dr. Kutluk Oktay of Cornell University tested two breast cancer treatment drugs - tamoxifen and letrozole - to try to blunt the estrogen spike.

A group of 32 breast cancer patients got either tamoxifen alone or tamoxifen or letrozole plus a low dose of the stimulating hormone. Thirty-three other breast cancer patients not seeking to freeze eggs were used for comparison.

Women who got either drug plus the hormone produced three to four times more eggs than those who got tamoxifen alone.

Letrozole seemed to be better, because it also kept estrogen levels low, Oktay said.

Longer follow-up is needed, but after a year and a half, the breast cancer recurrence rate was similar - three women in the comparison group and three in the drug group developed tumors. None of the women on letrozole did.

“It appears this is a safer method,” said Dr. Marian Damewood, a University of Pennsylvania obstetrician who heads the infertility group and had no role in the research.

Letrozole is sold as Femara by Novartis, though no drug company funded the study - women paid for their own treatment.

Meanwhile, doctors reported cautionary developments in using ovarian transplants to overcome cancer-related infertility. In this approach, tissue is removed and frozen before cancer treatment and transplanted back after treatment ends. Eggs are collected from the tissue and fertilized in a lab dish, and the resulting embryos are implanted to try to achieve pregnancy.

Usually the ovarian tissue is given just standard microscope evaluation to look for cancer cells. But Dr. Dror Meirow of Sheba Medical Center in Tel Aviv, Israel, reported that DNA analysis revealed signs of previously undetected cancer in two cases - a 38-year-old Israeli woman with breast cancer, and a woman in her 20s with leukemia who had flown to Israel from the United States hoping to have a transplant.

“She was disappointed, but she was sent back,” Meirow said. “It’s not enough to have a good reproductive center” to remove and freeze the tissue, he added. “The question is, is it safe?”

Certain cancers are more likely to have spread to ovarian tissue than others, said Dr. Marc Fritz, a University of North Carolina at Chapel Hill obstetrician who led the panel that developed the reproductive medicine group’s new guidelines.

The risk is high for leukemia, moderate for breast cancer and rare in most lymphomas and cervical cancers, “but there have been so very few such cases, it’s been next to impossible to quantify,” Fritz said.

Oktay, who reported the first embryo created from frozen ovarian tissue in 1999, said he believes the risk is very slim and not enough to justify the added cost of DNA testing.

Meirow said money would matter little to women who already defeated cancer once and would not want to face it again. The more sensitive DNA testing “is not done anywhere, and it should be mandatory,” he said.

Meirow’s research also has implications for young boys with cancer, who are not able to bank sperm before cancer treatment, as men can do. Testicular tissue has been frozen in some cases, but experts said they are unaware of any attempts to transplant it and restore sperm production in males after cancer treatment.