Women who become pregnant while dieting may increase the risk of their children becoming obese or diabetic, a major study has found.
A British-led team working with the University of Auckland's Liggins Institute made the finding in pregnant sheep, and its authors believed that the findings could be relevant to humans.
The discovery adds weight to New Zealand-led research that shows a baby's environment before its birth may determine its life-long health.
Liggins Institute research leader Frank Bloomfield said the study supported the organisation's argument that the most effective time to deliver public messages about health in pregnancy was before conception.
"Most women first seek medical care between six and eight weeks of pregnancy, and these data suggest that is probably too late for public health messages around maternal nutrition."
Researchers from the UK, Canada and New Zealand looked at pregnant sheep, some with one baby and some with twins.
Half of the sheep were fed a smaller amount for the first 30 days of their pregnancy, while the other half were well-fed throughout pregnancy.
The team then looked at the tissues in the brains of their lambs two weeks before birth to investigate if there were changes in their DNA structure.
Dr Bloomfield said that researchers found changes not in the genetic code, but in the expression of the genes that regulated appetite and energy levels. If these findings were replicated in humans, it would suggest that diabetes or obesity could be more likely in twins or in the children of mothers who were dieting or eating poorly at the time of conception.
He stressed that the data could not be directly applied to humans, but there was some human data which supported the findings. "If it's also there in humans then it is worthy of further research."
Dr Bloomfield emphasised that the level of under-nutrition given to the "dieting" sheep was modest. They were fed only 20 per cent less than the "non-dieting" sheep. "It would be comparable to someone actively dieting - it wasn't a starvation diet or a huge reduction in intake."
The study was significant because global rates of human obesity and type-2 diabetes were increasing. The rates of twin births has also risen in the past 30 years as women became pregnant later in life and increasingly used artificial reproductive techniques.
The occurrence of women becoming pregnant while dieting was also relatively high.
Dr Bloomfield cited British and Australian research which showed that up to a third of women of child-bearing age were actively dieting.
"The recommendation would be that women who are becoming pregnant, or thinking of becoming pregnant, shouldn't be actively dieting unless recommended by their healthcare practitioner."
The study was published in the Journal of the Federation of American Societies for Experimental Biology.
The editor-in-chief of the journal, Gerard Weissman, said: "This study shows that expecting mothers have to walk a really fine line when it comes to diet and nutrition.
"It also shows that epigenetics is the 'new genetics': both our DNA and the histones in which it is wrapped are susceptible to binge eating and dieting - we are what our mothers ate."
The British study has added to a growing body of knowledge that shows that a person's life-long health might be determined pre-birth.
A similar paper released late last year, which Liggins also contributed to, showed that the increased fat mass on adult sheep was determined very early in pregnancy.