As the sonographer stared at the monitor, biting her lip, my heart started to race. I was having my first scan and waited for her to tell me my baby was OK. It was Mum who punctured the silence. “Is it twins?” she joked. The lady turned and smiled. “Yes,” she said. “Identical.”

Mum screamed. I couldn’t tell if it was shock or excitement at first, but when she pulled me into her arms for a tight hug I knew she was happy.

I, on the other hand, felt like I was going to pass out. Dizzying thoughts were running through my mind. How was I going to cope? I already had two young children, Crystal, who was two, and one-year-old son Jay, to look after with my fiancé, Stephen Ellis.

It was February 2013 and it had been a roller coaster week. I’d been to see my doctor after getting achey pains in my stomach. He said my blood hormone levels were high and I must be around 14 weeks pregnant.

“I’m sorry?” I’d spat out, confused. I knew I was pregnant, but the date couldn’t have been right. I was sure I was only seven weeks.

The scan, at Singleton Hospital in Swansea, Wales, was to check how far along I really was and make sure everything was OK. Now I understood why my hormone levels were so high – I was having twins and I couldn’t have been happier.

Stephen, 29, a factory worker, was as shocked and delighted as I was.

A few weeks later, at the 12-week scan, I was amazed at how much the twins had grown. The two tiny shapes now had visible arms and legs. And, what’s more, they were girls.

Because the twins were sharing a placenta, Stephen and I were told it was a high-risk pregnancy and that I’d need a scan every two weeks. I wasn’t worried at the time, but six weeks later we found out just how risky it was. We were at a routine scan when, once again, the sonographer was looking intently at the screen. She called a consultant in who joined her in studying the monitor. I glanced at Stephen and saw the concern etched on his face.

“One of the babies is vacuumed in its sac. It can’t move,” the consultant finally said. “If you’d like to come to my office I’ll explain.”

Gripping Stephen’s hand, I followed the consultant. He told us the babies had twin-to-twin transfusion syndrome, or TTTS.

I listened, trying to take it in as he explained TTTS is a life-threatening condition found in twins sharing the same placenta. One twin was receiving all of the blood supply and becoming overloaded with fluid. As a consequence, she was at risk of a heart attack.

The other twin had very little fluid and so her sac had shrunk and stuck around her. She was also being squashed against the wall of the uterus and receiving very little blood. It meant she wouldn’t grow as well and might not survive.

I tried to hold back the tears, but one look at Stephen, my partner of nearly 10 years, and they came flooding out. I was so confused and scared. The consultant could see I was too upset to take anything else in. Instead, he told us about a specialist hospital that would be in touch.

We were on the way home when I received a phone call from Bristol’s Fetal Medicine Unit and arranged to visit them later in the week. They said to prepare myself for the possibility of surgery the same day. I was still only 18 weeks pregnant and terrified. Back home in Port Talbot, Wales, I tried to find out more about the condition. I was horrified by what I saw – websites littered with horror stories of women who had lost their babies or who were coming to terms with severely disabled children.

I had been looking for hope but it only escalated my fears.

Three days later, Stephen and Mum joined me for the appointment in Bristol. It was here that a surgeon sat us down and gave us the options.

“The first is terminating the pregnancy...” he said. I didn’t have to hear any more on the subject. I knew that wasn’t an option for me.

“The other option is whether to have surgery or not,” he continued. “Without surgery, the twins have 5 per cent chance of survival. With surgery, it’s 45 per cent.”

He went on to say there was a risk of complications even if they did survive. One or both of the babies could have mental or physical problems, such as cerebral palsy.

“I want the surgery,” I said. “I’ve got a feeling they can make it.”

We were then told what was going to happen. The surgeon wanted to perform a pioneering operation involving a laser beam to seal some of the shared blood vessels going to the larger twin so the girls would get an equal blood supply. What’s more, they’d do it while I was awake.

With a deep, shaking breath I agreed. And just minutes later I was being wheeled into theatre. My mum, sobbing, had to wait outside, but Stephen came with me. After a local anaesthetic, the surgeon made a small incision just off my belly button and then my womb. They inserted a camera and fibre optic laser down a tiny endoscope into my womb and started to examine the blood vessels going to the twins.

Awake, I could see the whole thing on a monitor. At first I couldn’t make anything out but then Stephen said: “Look! Look at that!” The camera was moving up behind one of the twins and I could see her tiny clenched hand on the screen.

“That’s incredible!” I gasped.

The surgeon started to use the laser to seal some of the blood vessels. It wasn’t painful but it was a little sore and I could feel a slight tugging inside as they worked.

After just minutes, the surgeon began stage two of the operation – draining excess fluid from the larger twin’s sac. They inserted a needle into my amniotic sac and carefully drained a litre and a half of fluid. After an hour in theatre, it was all over. “It went very well,” he said.

Ihad a scan after surgery and it revealed the smaller twin could already move again – fluid had started to enter her sac now that some of the larger twin’s vessels had been blocked off. It was amazing – they hadn’t even had to touch her during the operation. The surgeon explained there were still risks though. I could miscarry one or both of the twins and the chances of survival still stood at 45 per cent. But my babies had clung to life and so I clung to hope.

The following day, however, during a family get-together, I wasn’t feeling right. I had a nagging feeling, but brushed it off. I’d just had surgery, after all. “Come on,” Stephen said. “I’ll take you home to rest.”

At home, I lay down. I didn’t want Stephen to miss out on the party, so convinced him to head back to join our kids. Alone, I fell asleep. It must have been an hour later when I woke up to discover my waters had broke. Panicking, I was convinced it was the end, that I’d lost the twins. I was still only 19 weeks pregnant. How could they survive being born at such a young age?

Fighting back tears, I called Stephen and he rushed home to take me to hospital. “Everything will be fine,” he kept reassuring me.

Scans revealed the sacs had ruptured but the babies were moving. A consultant told us to expect the worst. My head accepted what he was saying. But as he was talking I could feel the twins kicking. “Maybe they can get through it,” I thought.

The next day I had a more detailed scan. It revealed that the twins’ sacs had resealed and filled back up. “Is that possible?” I asked the consultant. “I thought I was going into labour.”

The consultant was amazed, too. It does happen, but it’s incredibly rare.

I had to rest for the next fortnight. “There could be another rupture, and the outcome may not be the same. After that, though, it’s back to normal pregnancy,” the consultant said.

His words left me constantly worrying. Back at home, I discovered a TTTS Facebook support group. Dozens of posts were from mums who’d gone through a worrying pregnancy like mine to give birth to healthy babies. “Why couldn’t I have stumbled across this before, rather than the horror stories?” I thought.

At the next scan, two weeks later, I was told the babies were growing at a good rate. I relaxed slightly, knowing that every week that passed increased the girls’ chances of surviving.

It wasn’t until 26 weeks that I felt comfortable. I knew that if anything happened now the girls could be delivered and make it. After my scan at 34 weeks, I was called into the consultant’s office. I walked in having flashbacks of the horrible news that was delivered last time. “Please, don’t let there be something else,” I prayed.

“One of the twins is not making much progress. I think they’re ready to come out,” he said. I needed a caesarean, he explained, and four days later Stephen was by my side as I was wheeled into theatre.

It was another nerve-wracking moment, but as soon as I heard a baby’s cry I knew everything was going to be OK. All of the heartache and worry was worth it.

It was 3pm on September 24, 2013 when the twins were born. Lilly, the smaller twin, was delivered first weighing a healthy 2.26kg. Darcy followed two minutes later at 2.35kg.

I stole a glance before they were taken away and hooked up to oxygen to help their little lungs along. Stephen took Crystal and Jay to visit the twins and took lots of photos to show me. It wasn’t until midnight, after recovering from the C-section, that I saw them properly. They were perfect. My little miracles. And they looked exactly the same. It was only because they had their names on the incubators I could tell them apart.

Lilly and Darcy were in hospital for two weeks before they were allowed home; they had trouble feeding at first. Even then I was still anxious. But with every day that passed my fears slowly evaporated.

They’re a year now and still look exactly the same – Stephen can’t tell the difference! – but their personalities are starting to shine through. Being diagnosed with TTTS and then having their sacs rupture was a double scare but it was all worth it as they make the entire family twice as happy.