I have been overweight since I prematurely grew out of my sports kit at the age of 11. Diet regimes - slimming clubs, Weightwatchers, Rosemary Conley, high fibre, you name it - have come and gone with little lasting effect. As a teenager and in my 20s, weight dominated my thoughts. In my early 30s, at 18 stone, I was the fat bird no man wanted. Marriage, children, all thoughts of the family life I had imagined were sinking deeper into the bog of improbability.

But one night, out for a drink with friends, I met a man who saw a woman inside the fat bird. At 49, he was 18 years my senior. He had scraped his life back together after two failed marriages, with a child from each. We kept bumping into each other - at a party, the cinema. There was always laughter when we met; that and a comfort that seemed to come from an unspoken acknowledgement of each other's aspirations and limitations. We were accepting - me of his independence, he of my weight.

Eventually he invited me to dinner, and we slipped easily into a relationship. But I had fallen in love with a man whose needs were very different to mine - he didn't want a steady relationship, let alone cohabitation or children - and I kept wondering when it would end.

But it didn't. After a month he told me he loved me. A year on, he proposed. I accepted, of course, and we married in the warm South African sun, under the shade of plane trees, craggy blue mountains behind us. I had never been happier. Pete had reneged on his vow of independence because he loved me. But I was certain he would never reverse his decision not to have more children; he had even undergone a vasectomy. So my task was to hide my yearning for a child. Given time, I thought, these emotions would abate. But while I was suppressing my feelings, Pete was coming to terms with the idea of fatherhood again. It felt like the fusion of two opposing forces.

A few weeks after Pete's vasectomy was reversed, we were ready to start a family. We approached this task with vigour. But as the months passed, our once spontaneous lovemaking became more contrived as I was greeted by an ominous succession of monthly periods. I knew my weight might be contributing to our lack of success, so I returned to the round of diet purgatory. And, when the pounds wouldn't shift, I assuaged my frustration with food. I was caught in a fruitless, spiteful circle.

Neither of us could believe, though, that my weight was the only problem. A series of tests confirmed that the problem lay with Pete's low sperm count. Our only hope, our doctor said, lay in some form of assisted reproduction. He suggested a fertility clinic.

And that is how we found ourselves at the open evening of a top London clinic, where a tanned, confident consultant gave us and three other couples a tour of the laboratory and treatment rooms. We saw the huge stainless-steel freezer where sperm and embryos are stored, and the enclosed cubicle where the embryologist can inject a single sperm into a minute egg. We took coffee and gathered around the consultant for a final question and answer session. I asked him about low sperm counts and was heartened to hear that only one good sperm was needed.

I couldn't wait to get started. But in the same smooth voice, this polished professional calmly said that IVF might not be an option for me. The drugs needed to produce a good crop of eggs would not be so effective on overweight women. In addition, there were health implications for both an overweight mother and her baby.

Too fat to be a mother! Perhaps I should have expected this, but the smoothly delivered explanation slid through me like a glacial knife. I felt rather than saw other faces turn to me. Were they looks of sympathy or disdain? I didn't wait to find out: choking with tears and embarrassment, I left. The feeling of worthlessness was a physical ache that even Pete could not relieve.

Days later, after the anger had faded, we began to discuss other options. We agreed that I must lose weight. But how long would it take? I was 36. Soon age as well as weight would be against me. Finally, we decided I should attempt another diet. No snacking, no chocolate, no cake, and in the evenings we would take it in turns to scour the internet in search of a clinic with a high IVF success rate. But it seemed that the more successful clinics discriminated against overweight women. Born Hall, in Cambridge, announced on its homepage that "for health and safety reasons" it did not offer treatment to women with a body mass index - a measure of the ratio of height to weight - of 30 or more.

I was aware of my BMI. I felt as if the number 39 was burned on my forehead. Another crash diet followed. At the same time, the search for a clinic continued. The Essex Fertility Centre was repeatedly recommended by others in similar situations and we were heartened to see that no mention was made on its website of the dreaded BMI.

When the appointment came, I was quaking. We were ushered into a small, windowless office, and I could barely breathe. Pete said to take deep breaths and to try not to crush his hand. We stood across the desk from a tall man with wide shoulders and a soft voice. I remember thinking that bad news would not sound so bad if it were coming from him. He beamed warmth.

As he drew his explanations to a close, Pete gave me a long sideways glance. I knew that if I did not ask the question, he would. So I did: would there be any problems with my BMI of 39? I held my breath as an age crawled by.

Quietly and slowly, he explained that my weight was not ideal. It might make egg collection difficult, and could lead to problems in pregnancy. I had heard this before, and an aching weakness crept through my limbs. He continued. Plenty of overweight women fall pregnant, he said, and it was not his job to deny anyone the opportunity to try to have a baby. He would refuse treatment only if there were extreme physical circumstances, and my weight was not one of them.

Six weeks later I started a course of drugs to stimulate the growth of eggs. Pete was given a small phial, shown into an empty room, and told to use his imagination. We were delighted when the clinic called to say that six of my 10 eggs had fertilised. Our joy was uncontained when we saw the faint blue line on the tester. The fat lady was pregnant. Another fortnight passed, and we returned for a scan. We could see the pregnancy sac clearly. But the consultant's face looked grim as he told us there was no heartbeat. I do not think I have ever experienced such despair. I felt only very, very cold. I asked the consultant if my weight was to blame, but the only thing to blame, he said, was bad luck.

Weeks of discomfort followed. But we knew we had to try again. This time the blue line was bolder. The scan was different too. I could see the pregnancy sac and a tiny monkey nut, and I could also see a tiny pulsation at the heart of it. But more was to come. There was another monkey nut and another pulsation. We cried all the way home. I was having twins.

Max and Isobel were born two months early, after a whirlwind labour. I look at them now, in their 16th month. Max is a strapping thing who climbs and runs and smiles. Big-eyed Isobel sits demurely while attempting to pick Pete's nose.

As for me, I still struggle with my weight. I still feel the stares of disapproval. I should be used to people discriminating against me, but I wasn't prepared for such a brutal attack on my capacity for motherhood. My journey started before IVF became available through the NHS, and before the public debate over evidence that some NHS primary care trusts were discriminating against overweight women.

I was lucky. I could afford private treatment. I had the strength and support of family, friends and a dynamic, loving husband. But if the debate had started sooner, I may well have succumbed to an argument that attempts to create financial precedents for discriminating against women who already suffer discrimination in a multitude of ways. No one should be told they are too fat to try to be a mother

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