By Jane Elliott
Health reporter, BBC News
Victoria Shiells has two healthy little girls - but she knows it could so easily have been a different story.
Violet was born 10 weeks premature
For Chloe, now six, was born nine weeks prematurely and her little sister Violet was born 10 weeks early.
Experts are currently studying why women like Victoria give birth so early.
Around 43,000 babies are born prematurely in the UK every year, which is a major cause of infant death.
Studies have shown that nearly half of extremely premature babies who survive develop a disability or learning difficulty.
Another third develop mild impairments, such as the need to wear glasses by the time they reach six years old - double the average rate.
At present doctors have no idea why one in three premature babies are born early - and the problem is becoming more pressing as more and more babies are being born before the full 39-week term.
While both Chloe and Violet are developing well, their mother says that until they have some definitive answers she will not be risking a third pregnancy.
"My husband would like another one," she said.
"And if they knew what caused the early labour and if they could guarantee it wouldn't happen again, then I would think about it.
"But because they can't give me any guarantees, it has put me off having any more. It is just so stressful."
During her second pregnancy, Victoria was invited to join a study at King's College, London, and St Thomas' Hospital, to examine the role of infection in triggering pre-term labour.
The women were seen every fortnight throughout their pregnancy until their cervix shortened, preparing for labour. At this point they were then seen weekly.
"Everything was fine until I was 22 weeks pregnant," said Victoria.
"Then my cervix shortened and they said it had gone to the length it goes to when you go into labour.
"So from 22 weeks I was told to rest, which I tried to do, but having a six-year-old at home meant that was it hard.
"Then at 25 weeks I went into hospital for bed rest and my cervix had shortened even more. They did a test of my fluid in the vagina and it said I tested positive for going into labour within the next two weeks.
"So they put me onto bed rest in hospital. At 27.5 weeks I went home, but then at 30 weeks I went into spontaneous labour."
Luckily Violet was well. She had been given steroids before her birth to strengthen her lungs and just needed a bit of oxygen and antibiotics after her birth.
"She was only in hospital a month," said Victoria. "She is six months now and very small for her age, but doing everything she should be."
Hope for answers
Dr Rachel Tribe, who is co-ordinating and overseeing the Action Medical Research-funded study attended by Victoria, is hopeful it will provide some explanation for why some women seem particularly vulnerable to premature delivery.
It is estimated that 30% of women who have a pre-term labour, will go on to repeat the experience.
"We know a proportion of pre-term labour is linked to infection and we are trying to find out where it comes from and how it gets to the uterus," said Dr Tribe.
"One idea is that there is some sort of infection in your vagina that can shorten your cervix by triggering changes and as a result the infection goes into your uterine cavity.
"But we don't know the precise order of events. In some women the cervix may suddenly shorten and allow bacteria to enter the womb.
"Or it might be that infection and inflammation in the vaginal tract cause the cervix to shorten in the first place," she said.
"We can't predict who is going to have a pre-term labour. All we can use is past history."
Dr Yolande Harley from Action Medical Research said studies like this could help save lives and prevent disabilities.
"Premature birth is the single biggest cause of infant death in the UK every year," she said.
"Unfortunately doctors don't know what causes babies to be born too soon and there are only limited ways of slowing labour once it starts.
"What is known, though, is that babies stand a better chance of survival and less chance of serious long term complications if they stay in the womb for as long as possible."