'I'M PREGNANT.' The chill these words sent through me had nothing to do with the fear of impending parenthood. It was the fear of the only child, who, after 20 years, had just discovered she was about to lose her status as the centre (or so I told myself) of her mother's universe.

'Darling? Are you there?' I stood in the sweaty university telephone booth, trying to think of some enthusiastic comment to reassure my mother that yes, I was extremely happy for her, and yes, I couldn't wait to have a little brother or sister after all this time. Instead I croaked: 'Are you sure?' adding weakly: 'Will you still love me best?'

It was my first week at university. My mother was 40 and had given birth to me at 19. Ironically, I had spent the last week at home with her experiencing the most sustained pregnancy scare of my own life.

For three days she had endured my tears, self-recriminations and pleas for advice with an admirable calm. She convinced me that no problem was insurmountable, produced contingency plans, and eventually relaxed me sufficiently for my body to decide the scare had gone on long enough. Her generosity and open-mindedness was forgotten in the phone box. I burst into tears.

A week later, having received letters from her and her husband, Sandy, reassuring me that yes, they still loved me and were as shocked by the situation as I had been, I felt a little more confident, and told some of my new college friends.

'Wicked' was the general consensus, although some were astute enough, looking at my pained features, to ask: 'How do you feel about it?' 'Fine,' I lied. How could I explain the pangs of jealousy, the resentment towards this unborn usurper, the terror that somehow the creation of this new family would leave me, the child of the previous marriage, out in the cold?

I was being ridiculous, of course. As the weeks went by, my mother and Sandy humoured my erratic phone calls and tearful declarations of insecurity, and I gradually realised that exclusion was not going to be the problem. Even worse, they seemed to want me to be involved.

'You can come and babysit for me,' she smiled. 'Look, come and feel the kicking.'

Visiting only once every six weeks meant receiving a jolt every time I saw the rapidly increasing circumference of my mother's waist. When it became dome-like I got squeamish and couldn't look anymore. I mean, that was my mother. She wasn't meant to be that shape.

During one of her visits to my university, I became uneasy on a new front. She looked unwell. Although only six months gone, she looked puffy and moved as though much more heavily pregnant. Now I had new grounds for resenting this baby - it was making my mother ill.

Shortly afterwards, I was called out of a seminar to find a grey-faced Sandy in the corridor. He gently told me that my mother had been hospitalised, and that they had been told that the baby might not survive because of a blood condition called hydrops foetalis. Antibodies in my mother's rhesus negative blood had turned against the baby's blood and had sent it into heart failure.

Crying in the car park, I told Sandy that I hated this baby for the suffering it had brought my mother. I was shocked to hear him say he had felt rather ambivalent about it for the same reasons.

But when I heard later that day that my mother was being rushed to another hospital because they had identified how to save the baby, something changed, and I started willing it to survive.

Three weeks of driving around the country ensued as my mother was moved to different hospitals during the baby's fight for life. One of the last times I visited her, she went into labour and I sat gripping her hand for more than an hour as she waited for the powerful labour-suppressant drugs to take effect.

It would be nice to say that I was a tower of strength, but I spent half the time hyperventilating and the other half shouting for the nurse, having misread the monitors, while my mother, through clenched teeth, tried to comfort me.

Nine weeks early, my brother was born, and immediately placed in intensive care. I drove to the maternity hospital in Cambridge in the watery April sunshine, unsure of what I was going to say. Even more terrifying, I didn't know how I was going to feel.

The sight of him shocked me into silence. He weighed five pounds, two of which were excess water, a by-product of his antibodies' heroic attempts to fight back. And he was an orangey colour from the infrared light keeping his body warm. And unbelievably tiny and frail. The feelings that swamped me were shockingly protective. He was, after all, my brother.

Guy John Sanders was three last week. Not living at home means I don't have to share my things with him, which is no doubt conducive to sibling relationships. He also has an annoying propensity to talk about my boyfriend more than me, but I forgive him, and plan to bribe him into submission with Thomas the Tank Engine.