It is still unknown why about 40,000 UK women a year give birth prematurely
Survival rates for premature babies have improved substantially in the past 30 years but there has been much less progress in predicting or preventing premature births.
Sarah's daughter, Isabel was born three months premature in 2001. Her first few days of life were traumatic for both her and her family.
"She had a massive brain haemorrhage on her second day, which left long-lasting implications for her health.
"She has cerebral palsy, hydrocephalus, epilepsy, learning difficulties and a degree of visual impairment," said Sarah.
Sarah said when she was at the intensive care unit for pre-term babies, there was a strange atmosphere, as all the tiny babies on ventilators could not cry.
The outlook for Isabel, 8, was bleak, but she has done remarkably well, and loves going to school, Sarah said.
But Sarah has never found out why she went into labour so early.
More than 50,000 babies are born prematurely each year in the UK.
Pre-term is defined as being born before 37 weeks gestation, and the condition is still not well understood.
At the Tommy's Centre for Reproductive Biology in Edinburgh, Professor Jane Norman studies what makes a woman go into labour.
She said: "We can put a man on the moon but can't understand the very basic processes by which we all came into the world."
In 1975, half of all babies born too soon, weighing less than 1.5kg (3.3 lbs) died.
However now 90% of the premature babies who weigh 800g or more - about the same as a bag of sugar - survive.
The medical treatment for premature babies has improved dramatically.
The majority of pre-term babies are actually born between 32-36 weeks gestation and most will not need neonatal intensive care and will do well, with no serious health issues.
But for each week a baby is born before 32 weeks, there is an ever greater chance of serious complications soon after birth and of long term difficulties as children.
At St Thomas's Hospital in London, obstetrician Professor Andy Shennan runs a pioneering clinic to help women who are at risk of giving birth early.
It is known that 20% of women go into labour early because they are having twins or triplets.
But still that leaves about 40,000 women a year who go through a premature labour without an explanation.
"It's a massive impact on the national health service, not only immediately as the little babies need intensive care, but also longer term because some of the babies will have disabilities that will need the health service in later life," said Professor Shennan.
Between 25-33% of early births are induced because the baby is not doing well or the mother has developed a condition, such as pre-eclampsia.
Pre-eclampsia causes a pregnant woman's blood pressure to rise excessively, which can be dangerous for the baby and mother-to-be.
Professor Shennan's research has shown that there are many reasons for premature labour, from infection to the shape of the woman's womb.
Although occasionally drugs, alcohol or violence can bring on an early labour, most of the women are living healthily during their pregnancy.
Despite not knowing all the causes, Professor Andy Shennan has ways to predict who will go into labour early.
Kirsty was one of his patients, and had two premature babies - one at 29 weeks who survived and another at 25 weeks who did not.
When she became pregnant again she went to see Professor Shennan.
He put a stitch into her cervix, as the neck of her womb was short.
Kirsty's daughter Evie was born healthy at 37 weeks.
"It was so great to be able to hold her as soon as she was born. The other babies were in incubators," Kirsty said.
A study by Oxford University's Health Economics Research Centre estimated that the cost to the NHS of pre-term babies born in 2006 in England and Wales, would be £939m from their birth up to their 18th birthday.
Two thirds of these costs are related to a baby's initial stay in hospital.
The centre's research has suggested that if pre-term babies were born one week later the NHS could save £260 million a year.
But there is also a huge human cost.
The families of some of the babies who are born early soon realise they may have to face daily problems.
Every couple of weeks Isabel, 8, visits a physiotherapist who helps her walk with special boots and splints.
But mother Sarah is proud of her daughter's determination to walk longer distances.
"When we left the neo-natal unit all the indicators were that she would never walk."
A moment too soon is broadcast on BBC Radio 4, Monday, 22 June at 1100 BST.