Boys 'more likely' to be born early
Boys are 14% more likely to be born prematurely than girls, according to new figures, which show an extra 5,700 boys are born early each year in the UK.
Data for 2012 reveals there were 34,400 boys born under 37 weeks in the UK, compared to 28,700 girls.
Boys are also more likely to suffer death and disability as a result of being born too early, according to the new analysis.
Professor Joy Lawn, a neonatologist and epidemiologist at the London School of Hygiene & Tropical Medicine (LSHTM), led a global study on premature birth, which she said was a major problem around the world.
The figures showed the rate of premature birth remained largely unchanged in the UK at 7.8%, compared to around 5% in Scandinavian countries and 12% in the US.
There are 1,300 deaths due to complications from premature birth each year in the UK, mostly among babies born under 28 weeks.
Across the globe, boys have a 14% higher risk of being born prematurely, the research found.
Prof Lawn said: "Baby boys have a higher likelihood of infections, jaundice, birth complications, and congenital conditions but the biggest risk for baby boys is due to pre-term birth.
"For two babies born at the same degree of prematurity, a boy will have a higher risk of death and disability compared to a girl.
"Even in the womb, girls mature more rapidly than boys, which provides an advantage, because the lungs and other organs are more developed.
"One partial explanation for more pre-term births among boys is that women pregnant with a boy are more likely to have placental problems, pre-eclampsia, and high blood pressure - all associated with pre-term births."
Prof Lawn said boys had a biological predisposition to being born early.
"In the UK, an extra 6,000 boys or so are born pre-term each year in the UK," she added.
The studies found higher rates of disability in boys across a range of health problems, including cerebral palsy, blindness and visual impairment and effect on school performance.
"If you are born premature, even that little difference in maturity between girls and boys can make a big difference - particularly breathing complications for boys," Prof Lawn said.
She said both young mothers and older mothers had a higher risk of premature birth, with older mothers experiencing higher risk of high blood pressure, diabetes and medical complications.
IVF had also had an impact, increasing the number of multiple births, which were more likely to be premature.
Globally, the studies published in the journal Pediatric Research, showed that of the 15.1 million babies born too soon, one million died due to prematurity.
Of the survivors, 345,000 (2.7%) had moderate or severe disability.
In some countries, where girls receive less nutrition and medical care, girls were more likely to die than boys, despite this biological survival advantage for girls.
The research comes as Professor Andy Shennan, who runs the biggest premature baby clinic in the world, launched a new network to stop the "postcode lottery" in how pregnant women are treated.
He said there was currently no UK-wide guidance for detecting and treating women who might go into labour early.
This means a postcode lottery exists, with women in different parts of the UK receiving different treatments and various tests for predicting if they will go into labour early.
Many hospitals also use different thresholds for which women they are willing to treat.
Prof Shennan, who runs the clinic at St Thomas's Hospital in London, has now created t he Preterm Prevention Network, bringing together more than 30 specialists at hospitals and universities.
He said: "Preterm birth is a major public health issue similar to smoking, obesity or alcohol, yet, despite the profound impact it can have on both families and babies, it has not received the attention it deserves.
"Preventing prematurity could save thousands of lives and save many children from long term-health problems and, although there has been major progress in predicting and preventing women at risk from giving birth too soon, we still don't have an agreed national strategy that will help reduce the numbers.
"This problem has been seriously neglected compared to many less important areas in pregnancy. It is simply no longer an option to do nothing."
NHS costs and other costs for caring for premature babies exceeds £1 billion a year.
One in four babies who are born extremely prematurely in the UK have a disability such as cerebral palsy, blindness or profound hearing loss.
For babies born at 26 weeks and under, the average stay in hospital is almost 16 weeks.
Jane Brewin, chief executive of the baby charity Tommy's, said: "More than a thousand babies born prematurely lose their lives each year and, with prematurity rising, it's absolutely vital that we find a way of giving all women the best possible tests and treatments to reduce their chance of having their baby early.
"Over the last decade, we've been able to save more and more babies who are born very early, but there has been little change in the numbers who have lasting health problems.
"The only way forward is to all work together to find the causes and solutions, and I hope that the Preterm Prevention Network will help us get there in months rather than years."
In about 40% of cases of premature birth, there is no known cause.
Around a third of premature births are thought to be caused by infections in the womb but problems with the cervix can also cause premature birth.
If the cervix is less than 0.8in (2cm) long, women are more likely to have a baby early.