Week 1

I am lying on an examination table in a fertility clinic in Greece. The female doctor, syringe in hand, turns to me. "So, how many embryos should we put in?" she asks.

I have no idea what to say. I run through scenarios in my head. Twins, triplets, quads. No, definitely not quads. My brother Nick and his wife Jane, for whom I am to be a surrogate mother, would surely be delighted by twins, or would they? They're not here.

The doctor continues: "How do you feel about selective reduction?" she says.

Bam, the question hits me with a moral dilemma as real as a slap to the cheek. Selective reduction means aborting one or more foetuses to ensure the healthy delivery of remaining ones. It is common practice in IVF, but can I really do it? A multiple pregnancy can be dangerous, especially at my age (48). So, in the UK, there is a cap on the number implanted during IVF. However, the more embryos you put in, the greater the chance of success. At foreign clinics such as this one, they can sprinkle fertilised eggs like hundreds and thousands on a fairy cake.

It's a heck of a decision to make at the moment of conception, and these babies aren't even mine. They belong to Nick and Jane, although we've actually used his sperm and anonymously donated eggs to create the embryos.

There are two types of surrogacy: "straight surrogacy", where the surrogate mother uses her own egg and the intended father's sperm; and "host" or "gestational" surrogacy, which is more common, where the surrogate uses the intended mother's eggs, or donor eggs, and the intended father's sperm. Straight surrogacy is usually achieved through artificial insemination, and the baby has a genetic link to the surrogate mother. With host surrogacy, an egg is fertilised and transferred to the surrogate mother's uterus using IVF. In this case, the surrogate mother has no genetic link, and both intended parents can be named on the birth certificate.

At 45, Jane's eggs were judged potentially less successful than those belonging to a 25-year-old donor. In Greece, donors are anonymous, although you can specify eye and hair colour. This presents another moral dilemma. Is it OK to pay for eggs, even when the egg donor gets €3,000 – a fortune in economically ravaged Greece?

But then IVF, as I know from watching Nick and Jane struggle for the past year, is an unsentimental process. You grit your teeth and get on with it. My priority is to deliver a healthy baby to my brother and his wife. I tell the doctor that, yes, I do believe in selective reduction. She nods and suggests three embryos. The syringe goes in and I turn my head to watch on a fuzzy TV screen as the eggs are carefully placed on my womb lining, like three peas on to a soft cushion.

Week 2

I am now going through what is called the "10-day wait". This is the gap between egg transfer and the moment I can do a pregnancy test. It's a bit like waiting for exam results, except that the result matters even more because others are relying on me. After nine IVF attempts and two miscarriages, I can feel Nick and Jane's anxiety.

There are no figures for the number of surrogate births in the UK, because most are private arrangements. Commercial surrogacy, where the surrogate mother is paid, is illegal in the UK, although it is legal in some states of America, the Ukraine and India. Here, only "expenses" – typically up to £15,000 – can be paid. The first known case in the UK was in 1985 and since then it is estimated that 750 children have been born as a result of surrogacy.

Week 3

I'm pregnant. I am both elated and frightened. I already have two children, so I was confident I could get pregnant, but I am shocked it was so easy, especially when for others it is so difficult.

For a year, if I opened one of the drawers in Nick and Jane's fridge, all I saw was pills. Their lives were dominated by the chemical suppression of her menstrual cycle, which triggered depression so bad I hardly recognised her, and stimulation, which offered hope. If she wasn't swallowing capsules, she was injecting herself. Then there were dawn flights to Athens for egg collections and egg transfers, followed by the emotional hangover of each failed attempt and the even more devastating effect of the miscarriages. Jane did a pretty good job of covering all this, but it was terrible to watch. As for Nick, I could see him swallowing his own disappointment so as not to make Jane feel worse.

But then, I seem to be part of an IVF generation. I know at least four couples going through IVF. For Nick, it was that classic modern scenario of Jane having two children from her first marriage, meeting Nick, who at 38 is younger, but then not being able to get pregnant with his child.

Some would say the planet is already overpopulated, but when you watch people you care about being so unhappy, it is hard not to want to help. The tipping point for me came when I asked Nick straight out what he would feel if he couldn't ever have his own kid. He's a great stepdad, but he said, quietly and firmly: "I need my own child."

So, I offered. Jane refused at first. It was Nick and I who had to talk her round. I think she felt it was too much to ask of me, and it might complicate our relationship. Now, it's actually happened. We are on our way.

Week 4

I have one IVF friend in particular who has been having treatment on and off for 19 years. She is the fount of all knowledge on the subject and she suggests I go to a slightly grim Medicentre in Victoria station for a couple of blood tests. I then input the results to a fertility website and wait while it calculates my chances of a multiple pregnancy. If it's triplets, I'll need to confront the whole selective reduction thing, which I am dreading. It's not, it's twins. Nick is thrilled. I am slightly appalled at the idea of carrying two. I'll look like Mr Beefeater in a couple of months.

Week 5

I had forgotten that I actually don't like being pregnant. I am not the sort to rub her belly and light a scented candle. I feel rubbish and I'm not even getting a going-home present at the end.

Week 6

We have agreed not to tell anyone until we are past the 12-week mark. Still, I think I owe it to my children to explain my Wicked Witch of the West demeanour. I start with my son, 10. He frowns. "So who's the mum?" he asks. It's a fair point. These babies actually have three mothers: egg mummy (the donor), tummy mummy (me) and proper mummy (Jane).

This raises a tricky question: what if the babies want to find their biological mum? I tentatively broach the subject with Jane. How would she feel if I wrote to the clinic asking if the donor would be willing to leave some contact details for the babies to access when they are 18? Jane answers immediately: "No."

I tend to forget that it is hard for her to come to terms with the fact that not only are the eggs not hers, she is not carrying the babies either. She needs some territory of her own, I think.

Week 7

Is there a pregnant superheroine? If so, her superpower must be smell. When someone sits down next to me on the tube with a paper bag of chips, I almost throw up.

The drugs aren't helping. The clinic has given me oestrogen, progesterone, antibiotics, aspirin, folic acid and a steroid. I have a permanent headache and feel so tired that I am now taking a nap in the afternoon. The worst thing is the depression. I am normally a positive person, but I feel genuinely bleak. I ring my IVF guru. Is it normal to feel so depressed on these drugs, I ask. "Oh, yes," she says. "I waited 15 years to get pregnant and then I wanted to kill myself."

Week 8

The sickness is getting worse. Why did I offer to do this?

I go to Harley Street for an early scan. The babies are lying one on top of the other, as if on bunk beds. I suddenly feel quite maternal. "They are my babies," I think.

Week 9

I have been looking at due dates. I do the maths and discover that 38 weeks, the stage at which they will want me to deliver, is Easter weekend. Hmm, do consultants work over the Easter break? I doubt it. The thought of a dodgy locum doctor leaving a rubber glove inside me, then me catching MRSA and dying suddenly occurs.

Week 10

I meet up with a friend whose husband has walked out on her after years of IVF (yes, another one). She sobs: "I'm going to be 50 with no children." I am acutely aware of the two children I already have, plus the other two I am carrying. How do I tell her? I can't. Fortunately, she is so wrapped up in her own drama, she hasn't even noticed my expanded waistline.

I do tell another friend about the twins, though. He and his wife have also been having IVF. "Can I have one?" he says, only partly joking.

Week 11

This pregnancy feels very different from my others. I do not have a nursery to decorate, a pram to choose or even clothes to buy. Still, my grumpiness is abating slightly as I am now reducing the dose of the drugs.

The reaction of others has also been unexpected. They have been genuinely overwhelmed by what they see as my kindness. It has made me see myself in a different way. I am not a natural candidate for the Mother Teresa Memorial Award for Sainthood, but maybe I am not as selfish as I thought. After all, others may think about being a surrogate, but I am actually doing it.

Week 12

My daughter, who is six, has decided it is time to spread the good news. She went into school this morning and announced to her teaching assistant: "My mummy has two babies in her tummy for Auntie Jane because her tummy isn't working properly." The TA's mouth practically fell open.

I go back to the Harley Street clinic to see if we can find out the sex of the babies. They now have proper little faces and fingers and toes. The doctor looks at the first one. He's pretty sure it's a boy. I make a silent prayer that the other will be a girl (Jane already has two boys). It is a girl. I walk out of the clinic feeling high.

Week 13

I show the latest scan picture of the twins to my daughter. She says, as if I am a family pet having puppies: "Can we keep the girl?"

Week 14

I go to the hospital where I will give birth for a scan. I am surrounded by lots of lovely couples, looking excited. I suddenly feel very much on my own. When I get home, I burst into tears. I have been holding on so tight to my emotions, trying to remain detached, that I haven't allowed myself any room to feel anything. I watch Hermione Norris in an ITV drama about the mother of a murderer, sobbing.

Week 15

Jane told her kids today. Her eldest's reaction: "Why can't we be a normal family?"

Week 16

I phone my mother to tell her. "I am pregnant," I say. There is a long pause at the other end of the line. I decide to plough on with the full explanation. Finally, my mother rallies. "Well, despite the complications, it is lovely to have two new grandchildren," she says. "I just wish I was 10 years younger."

The way I am feeling at the moment, so do I.

Week 17

I don't know how Jane is really doing, because she has been keeping her distance. But I do wonder if she is underestimating how difficult this is for me (and, yes, I do know how me, me, me that sounds). She said to me this week: "People keep asking me how you are coping, but I tell them the thing about you is that you are able to compartmentalise." The babies are inside me, for goodness sake. That's a heck of a compartmentalisation.

Week 18

When I offered to become a surrogate, I asked myself one question: "Can I go home from the hospital without the babies?" This seemed for me the nub of it all. If I couldn't do that, I couldn't be a surrogate. I decided that I could and, having made that promise, I know I have to honour it. However, it is getting harder.

I found myself in a branch of JoJo Maman Bébé this week, looking at a soft pink bunny rabbit with the sort of dangly ears that small babies love to wrap their fingers around. In my other pregnancies I would have just bought it, but with this one I found myself wondering: "Am I allowed to buy this? Would it be wise, would it upset Jane, would it make it more difficult for me to walk away?" I walked out of the shop without it.

Week 19

Another scan. The babies look absolutely perfect. Still, I wonder if the doctor thinks I'm a bit weird. She keeps saying: "Do you see the hands… Do you see the heart… beautiful!" I nod and smile, but am muted. When they see that I already have two children and no apparent partner, they think I am very odd. One nurse says: "You like big families?" Another simply asks: "Are you Catholic?"

Week 20

I have Nick and Jane's son for a sleepover. We are talking about next year's summer holiday and I say, "Well, you'll have these," pointing to my tummy, "with you next summer." He looks nonplussed. "The twins," I say. "They're in here now, but I'll be handing them over to you at Easter." It has clearly not occurred to him that he is about to have a new brother and sister.

My own kids are now acutely aware of my pregnancy and have taken to whistling through their teeth like builders sizing up the cost of redoing a dodgy kitchen extension. "You're huge," my son says. But Nick and Jane's kids are not presented with this on a daily basis, so are in denial. I fear there may be trouble ahead.

Week 21

The Duchess of Cambridge is pregnant. She has been admitted to hospital with extreme morning sickness. Brilliant, while I look more whale-like by the day, Kate is going to remain a size 10 throughout.

Week 22

As we are leaving for school, my daughter turns to me and says: "You wouldn't give me away, would you?" I have been waiting for this question. I tell her "no", but it underlines the fact that, while the decision to offer myself as a surrogate was mine, the effect is on the whole family.

Week 23

One of the strange things about this pregnancy (and there are so many) is that it has operated as a time machine. Courtesy of IVF, I have stepped back a decade, rejoining the women-having-children club. Of course, I don't want any more kids of my own, but I am rather enjoying the anti-ageing effect of being seen to be able to reproduce.

Week 24

Christmas is in full swing, as are my pregnancy hormones. My daughter is doing a reading at the school carol concert. She freezes and I, normally Ms Rational, burst into tears. Another mother hands me a tissue with a look on her face that says: "Calm down, dear."

Week 25

Christmas Day. Nick asks me, jokingly: "How are my babies?" I feel annoyed. They are my babies, not his.

Week 26

Jane has begun discussing childcare. She is talking about hiring not one, but two nannies, because when she goes back to work, her hours will be long. I remind myself it is none of my business.

Week 27

I am now the recipient of random pity. Other mums at the school ask, "How are you?" with that tilted head you use when talking to someone with a particularly troubling illness. But then, I am so huge, I look like Violet in Charlie And The Chocolate Factory when she turns into a blueberry.

Week 28

It's snowing. I fell over twice yesterday. The second fall was a bad one. I am now worried that I may have hurt the babies. In the bath last night, I could see the girl, who is on the right, moving, but the boy seems very still. I'm not sure whether to go to the hospital and ask for a scan.

Week 29

The panic over the babies is over. I saw the boy move last night and was so relieved.

Nick and Jane have been to Ikea to buy a pair of cots. I felt a pang of sadness at that. Shouldn't that be me?

Week 30

I got an email from Nick today titled "Getting organised". In it, he told me he and Jane had found a specialist surrogacy lawyer and that it was quite a simple process for me to transfer parenthood of the babies to them. All I had to do was sign something and this would be presented to the court.

I feel pressured and rushed. I know Nick is just being efficient, but my emotions cannot be "organised". I email back that I am not signing anything (legally, I can't until six weeks after the birth anyway) and need some time to think.

I get an email from Jane: "You should never have agreed to do this if you were going to hold on to your status as mummy. You have broken my heart."

Week 31

I am now not answering my mobile to either Jane or Nick. I need time to think.

Week 32

A truce has been reached, courtesy of my mother, who has calmed down Jane and Nick. I've told her I don't want to keep the babies, but that I also don't want to sign anything either. I agree to meet Jane in Selfridges to choose going-home-from-hospital outfits for the babies. We eye each other nervously. She doesn't mention the contract.

I think my wobble was linked to the fact that the boy is still breech, so I'd been looking online for ways to turn him. I ended up seeing an illustration of a boy baby sitting up with his arms wrapped around himself. It connected in some way with me and I suddenly felt terribly protective of them both.

Perhaps it's better if I don't do any more internet surfing. I think I need to re-establish some distance between me and the babies.

Week 33

It is when the midwife says, "Get into the wheelchair" that I realise I am in trouble. My topline blood pressure is 196. The words "pre-eclampsia" have been uttered. "Are you worried I'm going to have a heart attack?" I say. "No, a stroke," she answers.

I spend the night on the antenatal ward listening to other women cooing over their newborn babies. I can hear their excitement. I feel sad.

Week 34

The consultant has pulled my caesarean forward by a week because of the pre-eclampsia. I have a conversation with Jane about how we are going to handle the birth. She is going to be with me in theatre, with Nick in the waiting room. My concern is whom they are going to hand the babies to when they are born. Jane says: "There are two babies, so we can hold one each."

Week 35

The team for the birth is huge: three anaesthetists, two paediatricians, a surgeon and numerous nurses. I look at them and realise this is serious. I feel calm. I turn to Jane, who is gowned up beside me, and say: "Last leg of the journey now. Just got to get them out safely." And that is how I feel, as if I am delivering something precious.

When they pull out the first twin, they take him away immediately, because he is a bit small. Then they ask Jane to cut the cord. The second twin appears and I suggest that Jane holds her. I am surprised I don't feel jealous. When they finally wheel me into recovery, my whole family is there. I get to hold the babies and I am amazed how little and perfect they are.

"Are you going to breastfeed?" a nurse asks. I look at Jane. I know she doesn't want me to and I was pretty rubbish at it with my own two, so I shake my head.

Week 36

I have to stay in hospital for 10 days, first because my blood pressure keeps going up and then because the babies are so small. They can't control their temperature and their weight drops, so we have to tube-feed them. Nick and Jane come every day and Jane stays to help me with the three-hourly feeds.

By the middle of week, I am heading for a burnout. I am exhausted and desperate to go home to see my own children. I have been away for a week. My daughter burst into tears at home with my mum this morning and my son is complaining of feeling sick and has stayed off school. They need me. We persuade the doctors to allow Jane to stay overnight so I can go home for 24 hours. Nick comes and takes me on the tube and I cry all the way. It is a combination of hormonal crash and tiredness.

The next day I go back to the hospital. One of the nurses says, "Mummy's back!" and I can almost see Jane wince. On Friday I go home again, but this time the trip is harder. I am acutely aware that this is a dry run for my separation from the twins. I get into bed next to my daughter, who is sharing my bed as a treat. As she sleeps soundly, I sob quietly beside her.

Going home from the hospital

On Saturday morning, I get a text from Jane from the hospital: "The babies are coming home today!" She is clearly thrilled. I am panic-stricken. It's too soon. I drop my kids with friends and get back on the bus. It is snowing again. Snowflakes hit the bus windows, melt and slide down the glass like big tears. I look out and my stomach is turning over and over in panic.

When I get to the hospital, everything has been packed. The paperwork takes ages and Jane goes out to get some food. When she has gone, I reach into the cot and lift first one and then the other twin out. I cry as I hold them close. I feel a great well of grief.

Jane comes back, Nick arrives and we push the pram out of the ward. When we get back to Nick and Jane's house, Nick opens a bottle of champagne. He is so happy. He and Jane toast the babies, and Jane turns to me and says, "Thanks." Tears bubble up again. I am aware of spoiling their big moment, so when Nick suggests he drives me home, I nod.

It's the moment I have been dreading for nine months. The moment I have to leave the babies. I bend over the pram and give each a last stroke on the cheek. I am trying to choke down my tears. With all the strength I can muster, I very deliberately turn and walk away. I can hear my own footsteps as I walk up the stairs and out of the front door. I pull it shut behind me.

Two weeks later

The first few days were tough. There was a great hole where the babies should have been, almost as if I had miscarried and they had died. I did a lot of crying. Having my own children around helped because it gave me something to do and little people to hug. Also, Nick and Jane have been generous and let me come and see the babies whenever I want. I think they also value the help, as twins are a lot of work.

It's getting better. I now no longer feel that the twins are mine. I can also see how much joy they have brought, particularly to Nick, and I feel proud of that.

If I knew how hard the journey would be, would I offer to be a surrogate again? At first I would have said "no". The last month of the pregnancy was physically gruelling and emotionally very difficult. Now, I don't know. In mad moments, I have wondered whether I should have another child for myself, but I know that's probably just the hormones talking. I have also wondered if I could do this for another couple.

Right at the beginning of this, I said to my children, "The right thing isn't always the easy thing." And I believe that. I have tears running down my cheeks as I write this, but it has brought my brother and his wife and I closer together. I did this for them and it has cost me in tears, but we are now bonded in a way that is fundamental and feels unbreakable.

My relationship with the twins is also special. They will never know what it cost me to give them up, but I know. I went this week to help with the six o'clock feed. The little girl was crying, so I picked her up and cuddled her. She quietened and fell asleep. "Do you think she remembers me?" I asked. Jane looked up, holding the other twin. "Of course," she said.

• Names have been changed. The author wishes to remain anonymous.