When patients consider a medical procedure, they may be told “there are no known long-term effects.” But unless such effects have been systematically studied, that does not mean there are no long-term effects.

That’s a major concern for Dr. Jennifer Schneider, mother of a three-time egg donor, Jessica Grace Wing. Ms. Wing was a tall, lean, attractive, athletic and musically talented Stanford University student when she decided to donate her eggs to help pay for her education. Through her multiple donations, five healthy children were born to three formerly childless families.

When her mother asked if egg donation was safe, Ms. Wing said she was told it was. What she did not know at the time was that no one had ever looked beyond the short-term effects of the many hormone injections needed to stimulate the release of multiple eggs at one time.

Four years after her third donation, Ms. Wing, then 29, learned she had metastatic colon cancer. Despite the best available treatment, she died at 31 in 2003, just days after completing the music for an opera called “Lost” that was presented three weeks later in New York.

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Ms. Wing’s cancer may have been totally unrelated to her egg donations. But given that Ms. Wing had been a health-conscious young woman with no family history of colon cancer or genes associated with this disease, Dr. Schneider wondered if the extensive hormone treatments her daughter had undergone might have stimulated growth of the cancer and if other egg donors might also be at risk.

Alas, she soon discovered, it was impossible to know because no one was keeping track of the medical or psychological fate of egg donors. Once donors walk out the door, they are essentially lost to medical history.

Dr. Schneider began vigorously advocating for establishment of an egg donor registry that could benefit not only the thousands of healthy young women whose eggs help others get pregnant, but also the growing number of women who want to postpone pregnancy and choose to have their eggs frozen for future use.

But 14 years after her daughter’s death, there is still no one tracking the fate of egg donors. The Centers for Disease Control and Prevention collects information on in vitro fertilization, but not on those who donate their eggs either anonymously or to family members or friends unable to get pregnant with their own eggs.

There are not even reliable data on the number of egg donors. The best statistic I could find was the number of donor eggs that were used for in vitro fertilization, which increased to 18,306 in 2010 from 10,801 in 2000.

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Fertility clinics and egg-donor agencies advertise broadly, especially on college campuses and even on New York subways, for donors, who are typically paid $5,000 to $10,000 for each donation, even more if the woman satisfies a fertility client’s specifications. One woman wrote in The Atlantic that she had responded to an offer for $25,000 in The Yale Daily News for “a young woman over five feet five, of Jewish heritage, athletic, with a combined SAT score of 1500, and attractive.”

Donor eggs are typically used for women seeking pregnancy who may have undergone early menopause, have poor egg quality, a history of genetic disease, a hormonal imbalance, ovaries that fail to respond to stimulation, or are over the age of 40.

“Egg donors are just not on anyone’s radar,” Dr. Schneider, who lives in Tucson and is certified in internal medicine, addiction medicine and pain management, said in an interview. “It’s not the same as sperm donation, which doesn’t involve hormone injections or any invasive treatments. In my opinion, egg donors need to be treated like all other organ donors — their health should be monitored.”

What we have now are only anecdotal reports of women who served as egg donors and later developed cancer. For example, five years before Ms. Wing’s untimely death, two doctors at a London fertility clinic described in the journal Human Reproduction the “tragic case of a young woman who died of cancer of the colon after successfully donating eggs to her younger sister.” They noted that long-term safety concerns about egg donation had been raised in the British Medical Journal (now BMJ) in 1989, given the high levels of hormones administered to donors.

The London doctors, K.K. Ahuja and E.G. Simons of Cromwell Hospital, called upon the British Human Fertilization and Embryology Authority “to confirm that ovarian stimulation in volunteer egg donors does not increase the risk of cancer over and above the background rate.” But this has yet to be done.

Earlier this year, in Reproductive Biomedicine Online, Dr. Schneider and two co-authors reported on five cases of breast cancer among egg donors, four of them women in their 30s and all five of whom had no apparent genetic risk for the disease. None of the women had been given any information about long-term risks of egg donation, because no such information exists.

The authors pointed out that single cases do not establish whether hormone stimulation of egg donors increases the risk of various cancers, and reiterated Dr. Schneider’s earlier pleas for “the need to create egg donor registries that will facilitate long-term studies on egg donors.”

Similar risks might also exist for women who are able to supply their own eggs for in vitro fertilization, or I.V.F. They undergo the same hormone treatments as egg donors. However, these women, who are typically older and may have endocrine or other abnormalities, are not a reliable group for determining long-term risks for young, healthy egg donors.

Dr. Schneider and co-authors expressed concern as well about another group of women: those whose eggs are harvested and preserved for their own future use. They undergo the same hormone treatments, perhaps more than once if not enough healthy eggs are obtained on a first attempt. If successful, egg freezing can enable women to delay pregnancy and remain in the work force until well beyond normal childbearing years.

Some companies, including Apple, Facebook, Google, Intel and Microsoft, now cover the cost of fertility treatments, in some cases including egg freezing, which can cost about $10,000 plus $500 to $1,000 a year for storage. Facebook, for example, is said to offer a $20,000 egg-freezing benefit for women employees.

But until more is known, Dr. Schneider has written, “All women who undergo ovarian stimulation, especially more than once, should be told that their long-term health risks are unknown.”