The Egg Donor Program, in Los Angeles, specialises in matching prospective parents with top-notch egg donors - young women selected for their beauty and brains, as well as their musical or other talents. At any one time, the agency has around 350 donors on its books. Unlike the UK, the United States allows women to sell their eggs - usually for $3,000-$5,000 (£1,700-£2,800), although sometimes for much more. As a result, agencies such as this one have no shortage of volunteers.

But despite the wide choice, parents are increasingly hard to satisfy. Shelley Smith, founder of the Egg Donor Program, says: "Sometimes they've seen thousands of donor pictures and out of all those, find just one possible match. Then they complain because her eyebrows are too dark." One woman, Smith recalls, flung a donor photo across the room, complaining, "Her lips are too big. What else have you got?"

Donating an egg is time-consuming and labour-intensive. Jen (not her real name), a clinical research scientist in San Francisco, has done so five times. For each donation, she injected herself daily with hormones for three to four weeks, as well as making frequent trips to the clinic for blood tests and ultrasound scans. Forty-eight hours before the egg retrieval, she injected herself in the back of her thigh with a drug that stimulated her ovaries to produce many more eggs than usual. At the clinic, she submitted to a general anaesthetic so a doctor could aspirate the eggs with a needle.

The process is not without risk. For one donation, Jen's ovaries produced an unusually large number of eggs and when they were retrieved, her follicles filled with fluid, causing painful bloating. This condition, called ovarian hyper stimulation syndrome, affects 4-5% of donors and can lead to infertility or even death. Jen recalls, "The doctor said my ovaries were swollen up to the size of softballs. I felt as if I was five months pregnant." Jen had to take a week off work, popping painkillers and wearing maternity clothes until her stomach subsided.

Although the money helped pay off student loans, Jen's primary motive was not financial: "I don't want kids, so this is my contribution."

But whatever its Darwinian appeal, egg donation is a big commitment to make for free. It's no surprise that in the UK, where one in seven couples has problems with infertility, 90% of clinics have an egg shortage. A law passed in April could make this worse: children born of donor eggs now have the legal right to know who their genetic parents are. This loss of anonymity could discourage donors, and paying them may be the only way to lure them back. Consequently, in its recent public consultation on egg, sperm and embryo donation, the Human Fertilisation and Embryology Authority raised the question of payment for egg donors.

Meanwhile, the European Parliament held a conference on egg cell trade last month, in part to discuss the downside of offering payment. Some fear the money may blind women in financial need to the medical risks of donation. Others argue that women are capable of making their own decisions, but worry the profit motive may drive clinics to understate the side effects. In the US there are no legal guidelines on what egg donation agencies have to tell potential donors, and they often do not offer adequate information. "Donors who have talked to other agencies come to me and they don't even know they're supposed to take shots," says Smith.

Despite its disadvantages, offering payment seems the only way to attract plenty of donors. But as the situation in America shows, having too many donors to choose from can create problems of its own. There, parents can more or less order an egg tailored to their specifications - the same way they would a coffee at Starbucks. On the website of the Genetics and In Vitro Fertilisation Institute in Virginia, for example, they can search the roster of 90-100 donors by race, blood type, hair colour, eye colour, height and education. This is in stark contrast to the situation in the UK, where parents usually get a donor who is a racial match, but otherwise have little say in the matter.

A decade ago, the situation in the US was similar. But in the mid-90s, clinics began offering donors $5,000 or more, and suddenly there were hordes of donors. In the past, parents usually found a donor through a fertility clinic. Then the egg brokerage industry was born: hundreds of agencies dedicated solely to match-making parents and donors. (Once a donor has been found, the agency refers parents to a clinic to take care of the actual egg retrieval.) The rise of the internet meant that brokers could make their catalogues readily available. Now, at the click of a mouse, people can search the databases of not just one but several agencies, giving them access to thousands of profiles.

But although parents eagerly pore over the profiles, they are becoming less and less interested in face-to-face meetings. When Smith launched her agency, in 1991, nearly one in four couples asked to meet the donor. Now it's one in 10. Smith herself got pregnant by using an egg donor (she now has 10-year-old twins) and she says it was meeting the donor that clinched her decision. She recalls, "It was about connecting with someone I liked. I wasn't insistent on a particular look or on having her be this way or that way." Now Smith's clients make their decisions based on college transcripts and vital statistics. "Rather than saying, 'She's a lovely person I connected with,' these days, parents will say 'She's got light-blue eyes and 1,230 Sats.'" In short, Smith says, many now treat choosing a donor like "shopping for a sweater at Bloomingdale's".

Victoria and Collin, a British couple living in London, were on a waiting list for a UK donor for two years before hearing about the Egg Donor Program in LA through a friend. "In Britain," Victoria says, "you wait years and then you have to take whatever donor you are offered. With the Egg Donor Program, we picked 10 we really liked from the catalogue. Then we went to separate rooms and chose our top three, ranking them in order of preference. Health was the most important thing. We also wanted someone who looked like me and someone fairly small, because I'm five feet and Collin is not too tall either."

Victoria and Collin ended up picking the same donor. Having waited so long, they were easy to satisfy. But those who have access to a large donor pool from the start are demanding more and more. Some parents have become so finicky that the donors offered by agencies don't measure up to their standards. Then they advertise in search of the perfect donor, often in Ivy League publications. The Stanford Daily News runs so many of these ads that a special section is reserved for them. One ad, placed in the Yale Daily News and several other Ivy League publications in 1999, offered $50,000 for the eggs of an Amazonian superwoman: "Candidate should be intelligent, athletic, blonde, at least 5ft 10in, have a 1,400+ Sat score, and possess no major family medical issues."

Some parents seek beauty over Ivy League credentials. In 1999, Ron Harris launched ronsangels.com, a website where couples could bid for the eggs of models in online auctions. Harris is a porn photographer. (On the site, he also boasts of his experience as an "Arabian horse breeder".) Some were outraged by the site; others said it was a hoax - nothing but a publicity stunt to promote Harris's porn business. Harris continues to insist that the site is genuine, claiming it earned $39.2m between 1999 and 2004.

Whether it was a hoax or not, shortly after Harris' site appeared, the American Society for Reproductive Medicine issued a recommendation that donors be paid up to $5,000 (and certainly no more than $10,000). The report decried paying high fees as "a form of positive eugenics". But parents who reproduce the old-fashioned way have the right to choose the genes. Why shouldn't infertile couples exercise the same right?

And if parents can buy music lessons for their kids, why shouldn't they buy musical genes? Ken Baum, a lecturer in bioethics at Yale College who has written on the subject, says it might be distasteful, but it isn't wrong. "The rich can pay for a more expensive car, they can get better healthcare and better housing ... In a democratic, capitalist society, why shouldn't we treat eggs like any other commodity?"

Some feel that treating eggs like a commodity devalues human life. Victoria says she finds paying high fees unethical. "I don't like it when money is the motive behind it. Many of the women in the catalogue were wannabe actresses who were obviously after the money to fund their careers." But Victoria is expecting a boy in a few weeks, and is well aware that without having paid a donor, she wouldn't be pregnant.

The real problem with the egg market may be psychological rather than ethical. Phyllis Martin is a licensed professional counsellor in Virginia, who treats individuals dealing with fertility problems, including those using donor eggs. She says that with so much freedom to choose, her clients find the decision overwhelming: "People walk out of the clinic, cry, or get angry and frustrated. It's not an easy thing to have all those donors before you." It's natural for someone to want to find the perfect egg donor but, Smith says: "The more information you give, the more opportunities they have, the fussier parents get."

Professor Barry Schwartz, author of The Paradox of Choice, agrees that too much choice can be more burden than blessing. "People have a hard time choosing cereal. I can't imagine how you make a decision like this, which is consequential and non-reversible. It would seem to me to be sheer torture. How do you figure out what is the best? Is it the most beautiful or the smartest? Or is it someone who looks most like the mother so no one will know?" Of course, even though parents invest so much in their decision, in the end heritability is a dice roll. Many of the traits that parents search for so desperately - a college education, for example, or even height - could be as much the result of nurture as nature. Phyllis Martin says parents overestimate the importance of the donor: "There's an awful lot you hand out to kids as nurturer that has nothing to do with eye colour."

The HFEA will announce the results of its consultation this autumn. If the organisation does recommend that donors be paid in the UK, the suggested amount - a maximum of £1,000 - is likely not to be large enough to draw hordes of donors, and clinics will have at most a small choice to offer. But that could be a good thing. Schwartz says, "From the research on ordinary things, the evidence is when choosing from a lot of stuff, you end up less satisfied. The dark implication is that when you choose a donor and have a baby, you're less thrilled than you would have been if you hadn't had so many options to choose from."