Mothers who have a high carbohydrate diet in early pregnancy and a low dairy and animal protein diet in later pregnancy may risk having smaller placentas and lighter babies, according to a new study of pregnant women.
The research, in today's British Medical Journal, shows for the first time that a mother's diet in early pregnancy can affect the placenta and the nourishment it gives the foetus.
But although a low birth weight increases the risks of an infant's chance of heart disease later in life, the study's researchers said there was not enough evidence yet for changing pregnant women's diets.
Dr Keith Godfrey and colleagues from the Southampton General Hospital in England studied the nutrient intake of 538 women during pregnancy who delivered at term.
They said little is known about how nutrient intakes in early pregnancy relate to placental and foetal size in humans.
And while nutrient intakes in late pregnancy have been found to have inconsistent effects on the size of the foetus, their effect on placental size is still largely unknown.
The researchers explained that the apparent paradox that high carbohydrate intakes in early pregnancy leads to lower placental and birth weights may be explained by a comparison with sheep.
In farming, it is common for ewes to be put on rich pasture before they are mated, and then on poor pasture in early pregnancy. Experimental studies on farm animals suggest placental growth may be stimulated by under-nutrition in early pregnancy.
The dietary associations in the women studied existed after the mother's own birth weight was removed from the equation.
A mother's birth weight is a strong determinant in what her baby's birth weight will be.
An obstetrician at the Royal Women's Hospital in Melbourne, Dr Michael Permezel, said if the study was correct, it could have implications for nausea in early pregnancy.
"Confirmation is needed, but perhaps women needn't feel as bad if they don't feel as hungry in early pregnancy," he said.
An epidemiologist at the Royal Women's Hospital in Melbourne, Dr Robin Bell, said the results seemed baffling and paradoxical and needed further investigation.
"It's very important not to leap to conclusions on the basis of parallels with sheep, because there could be issues related to metabolism that are different in sheep," she said.
An obstetrician, at Melbourne's Mercy Hospital, Professor Norman Beischer, said the study's conclusion on early pregnancy did not sound correct.
"In my opinion it's access to calories that decides birth weight, rather than types of food, although of course all pregnant women should have a balanced diet."