I had been considering donating my eggs for several years. I kept coming across stories in the newspapers or on the radio and TV about the heartbreak involved for couples who couldn't have children. I had a friend in the United States who went through six or seven IVF cycles without success and that was heartbreaking too, though in the end, wonderfully, she became pregnant. I thought: I'm 29 years old and suitable for this. I can help change someone's life for the better.

My family was really supportive. I was nervous about telling my parents because they're a different generation and attitudes have changed so much. But my Dad's exact words were, "Good for you!" My Mum and sister both felt the same. I'd found the website of the National Gamete Donation Trust (NGDT) and they, too, were fantastically helpful, not just with legal, ethical and medical information, but with their email forum. You register for this but it can be anonymous, although that wasn't an issue for me. I sent an email saying, "I'm thinking of donating" and got plenty of replies from women who had done it. They were very supportive without asserting pressure.

The trust provided the details of my nearest clinics. It was a very big step to contact them. You don't just think about it and next day call. I emailed Pauline, the egg donation coordinator, who is a senior nurse, and she invited me in for a chat. We had about 45 minutes together. She thanked me for considering donation and put me at ease. I know they're desperate for donors - there's a four-year waiting list for recipients in Leeds - but I didn't feel under any pressure. They absolutely do not want donors who are not certain about what they are doing. The message is: if you have any doubts you can pull out at any time.

The next step is having tests to match you with potential recipients; colour of eyes, hair, height and blood group. Then you have medical checks, screening for things like hepatitis and HIV, family medical history, hormone tests and an ultrasound scan of your ovaries and uterus. It's all done in one session.

Separately, I had four 50-minute counselling sessions and these were very thorough. The recipients get them too. The counsellor asks how your family feels, how you think you'd feel when or if you have your own children. There is heavy counselling about a donor-child's right to know who you are when they turn 18. I was seen by a woman for both the medical tests and the counselling and I'm sure you can insist on that. One of the Leeds counsellors is a man and so are several of the doctors and both of the consultants.

A few months later, the letter came from Pauline saying: "We have a match." Now it's real. There's a couple out there waiting for my eggs and they have waited a long time. I imagined how ecstatic and nervous and excited they must have been and felt even more sure I was doing something good for somebody. There is a 25% to 35% chance of pregnancy per egg.

I had another blood test to check my hormone levels and then they asked me to visit the clinic on day 21 of my cycle to start hormone treatment. The donor and recipient need to be synchronised so my cycle had to be "down-regulated" for 10 days. They gave me a nasal spray to sniff three times a day for 10 days. It was fine - in fact I went to Rome with some friends feeling perfectly happy. The day after my treatment began, the recipient went in to start hers. You never go on the same day. Of course I was curious about her but you never meet. You know that from the start.

After the 10 days, there was another scan and then they started stimulating my ovaries with two nasal sprays and one injection of FSH (follicle stimulating hormone) a day. You do it yourself at the top of your thigh - they show you how and give you tips, such as using an ice cube to numb the skin. It's how diabetics inject insulin. I was dreading it. The clinic and my GP said they would do it if I didn't want to but it was fine. Have a shower, do the injection, have breakfast, go to work.

There was another scan then a final, different injection of HCG (human chorionic gonadotrophin). FSH and HCG are hormones that women produce naturally, and are given to the donor to stimulate several eggs to grow rather than just one or two. I managed to produce 12, which I thought of as "12 good chances" for the recipient and her partner.

Egg collection takes place 36 hours later. They gave me a tablet to take the night before to calm me down, because you do get tense. I went to work the next morning to take my mind off it before showing up at the clinic and getting into my nightshirt.

The anaesthetist asked me: "Do you want to be completely asleep or just zonked?" I don't like the idea of a general anaesthetic so I opted for zonked. Extraction of the eggs using needles took 25 minutes. Afterwards I felt terribly tired, went home to bed and felt a bit tender round my stomach but only for a couple of days. I'd compare the effects to having a cervical smear. Or maybe overdoing an aerobics class at the gym.

I filled in a form for the recipient just beforehand. You can tell them as much or as little as you want. I described my interests, my view of the world, my family. I had logged on to an infertility forum first to ask recipients what they had found useful. And I took the option of sending a message to the child. It's hard writing a letter to someone who will not read it until 18 years later and I thought about it a lot. I explained how the child's parents had longed for him or her and how I was able to help. I also sent my own good luck card to the recipients, anonymously through the clinic.

I was asked in counselling if I felt I'd be giving up a baby. Definitely not. I am a biomedical researcher at Leeds University and see that this is an unfertilised cell. I would not think of myself as the child's mother. The recipient carries the child, gives birth to and makes milk for the baby and brings them up.

I would consider doing it again, especially for someone I know. I can claim for my expenses, such as two days off work, but that is it. I didn't do it for money, but perhaps the government should offer a financial incentive to encourage more donations. I will tell everyone involved in my own life - a partner, my own children if I have them - about my donation. I really hope my donation does result in a child.

· Sarah Harris was talking to Martin Wainwright. For more information go to www.ngdt.co.uk