Women who want to get pregnant should ensure that they eat properly before conception to minimise the risk of a premature birth, research suggests.
Prematurity can be linked to health problems
Scientists have found that even modest restrictions in maternal nutrition around the time of conception can lead to premature birth.
Prematurity has been linked to poor physical and mental development, in part because premature babies have less opportunity for their lungs and organs to develop in preparation for life outside the womb.
The researchers, from Canada, New Zealand and Australia, believe the finding may shed new light on the 40% of premature births for which there is currently no obvious explanation.
The new research was conducted on sheep.
Do not diet substantially before the start of pregnancy
Half were properly fed prior to mating, and throughout pregnancy.
The other half were underfed prior to mating, and for the first part of pregnancy before being allowed to eat whatever they wanted in the later stages.
The underfeeding was designed to make the sheep lose about 15% of their bodyweight - defined as mild to moderate nutrient restriction.
On average, the underfed ewes gave birth one week early.
The researchers found that lack of nutrients appeared to accelerate the development of the lambs' adrenal glands.
It is thought that the adrenal gland triggers birth by stimulating the release of hormones into the blood.
Premature birth is the number one cause of illness and death among newborn babies.
It has become more common in affluent Western societies over the past decade.
The researchers believe this may in part due to women going on a diet prior to conception, for fear of being unable to shed excess weight later.
Researcher Professor John Challis, of the University of Toronto, said: "Women need to think about proper diet and food intake before they even know they're pregnant because proper nutrition after pregnancy may not compensate for the lack of it beforehand.
"Even a modest restriction around the time of conception could have far-reaching consequences.
"We know now that this period of undernutrition could impact on the development of the pituitary and adrenal glands of the baby and may well affect the development of other organ systems.
"So if you're intending to get pregnant, do not diet substantially before the start of pregnancy because it's not necessarily a good thing for your pregnancy and you may cause your baby to be born prematurely."
Dr Frank Bloomfield, from the University of Auckland, who also worked on the research, said: "If you could reduce pre-term birth by some small percentage, you could improve the long-term health and lives of many people."
Research has suggested that up to 40% of women of child-bearing age in the UK do not eat a good, well-balanced diet.
The research is published in the journal Science.