Lying on the bathroom floor, I couldn't believe how ill I felt. I lifted my head over the bowl and was sick, yet again. Several weeks after Helen, my identical twin sister, had told me she was pregnant with her first child, I began to feel waves of intense nausea, often soon after I got up. It was different from any other sickness I'd experienced, and would come on violently and without warning. I knew it was impossible, but it felt like only one thing: morning sickness.

I had no idea Helen was going through the same thing - although we were close, we didn't talk to each other that often. She lives more than 100 miles away in Hertfordshire and I'm in London. It was a fortnight later she phoned to tell me her symptoms. They were identical to mine.

I see myself as a rational person and I've never had much time for stories about twin "connections". Helen and I had grown up listening to the same old questions that people ask when they see that you look the same: "Do your boyfriends get confused?" "Have you ever kissed the same boy?" We bore it with good humour.

Sometimes we have finished each other's sentences, and one year we sent each other identical birthday presents, but nothing of real significance. This felt different. Was it in my imagination? It didn't feel like it. I was about to go to my GP when the symptoms began to subside. It was a relief to feel normal again and I put this episode down to a lingering stomach bug or virus. Then, a few weeks before I'd been told Helen was due to give birth, the symptoms returned, and got worse.

I woke up at about 2am one Saturday with chronic stomach cramps. I spent the early hours running between the bathroom and bedroom, overwhelmed with waves of pain, panic and nausea. I even thought about calling an emergency doctor and wondered if it was something I'd eaten, then it began to calm down and eventually I fell asleep.

A few days later, Helen rang. She told me that at about the same time - 2am on Saturday - she, too, had had crippling cramps and thought she was going into early labour.

We couldn't believe it - I knew it must be a coincidence and yet, this time, I couldn't help feeling our experiences were linked.

Then, two weeks later, I was working late so I could take the next day off to be at the hospital for Helen's birth. Switching off my PC, I noticed the time, 7.51pm. I remember bending down to gather my bags from under my desk, then a gut-wrenching spasm in my lower abdomen. I buckled back in my chair. It passed after a few minutes, but I felt dizzy and disoriented. I made it on to the tube, still feeling horribly sick. I also had a sense of dread: did this mean something was happening to my sister, too?

When I got home, there was a message from my mum to phone her urgently. I called her and she told me Helen had given birth to a boy - at 7.51pm, just as I'd turned off my PC and collapsed in my chair. She'd had a healthy baby, but the feelings of dread now made sense to me as my mother told me what had happened - Helen's birth had been traumatic and complicated. The baby's heart rate had dropped dramatically when she went into labour and she had to have an emergency caesarean. At one point, it was touch and go. She had lost a great deal of blood and had to have a transfusion. My mum told me she was still in recovery. Her husband was sitting outside the theatre with their new son in his arms, waiting for her to come round.

As my mum described the scene, I cried uncontrollably, thinking of Helen. I felt now, without a doubt, that I had experienced her birth pains, and part of her trauma, too.

Worse was the guilt that while she lay there, I knew she had given birth to a beautiful baby boy and she didn't.

It took my sister a long time to recover. Neither of us could quite believe what we'd shared, even though we'd been miles apart. Physically feeling some of her pain put me in awe of what she must have been through and we both feel much closer as a result. At least I now know what pregnancy entails - I'm hoping for both our sakes she has an easier pregnancy next time.

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