A team of scientists compared the birth weights of babies born to mothers who drank a cup or more of milk a day during pregnancy with ones whose mothers did not. The babies of milk drinkers were heavier.

All 300 women who participated in the study, published in the Canadian Medical Association Journal, were non-smokers and followed conventional health advice during pregnancy. But a quarter admitted to drinking less than a cup of milk a day.

The research team leader, Kristine Koski, director of the School of Dietetics and Human Nutrition at McGill University in Montreal, Canada, said: “This is an important finding because increasing numbers of women are restricting milk consumption during pregnancy believing that it will lower fat intake, minimise weight gain, treat self-diagnosed lactose intolerance or prevent their children from developing allergies.

“Mothers and health professionals need to understand that this dietary practice may restrict essential nutrients and negatively affect foetal development.”

The women taking part in the study were aged 19 to 45 and attended antenatal classes at three hospitals in Calgary between May 1997 and June 1999. Of these, 72 drank less than a cup of milk a day.

A cup of milk was chosen as the benchmark because it contains about 2.5 micrograms of vitamin D. It is significantly below the daily intake of 5 micrograms recommended by some dieticians and nutritionists. Vitamin D is considered important in skeletal growth and in producing strong bones. It is needed to transport calcium from a woman during pregnancy through the umbilical cord to the foetus.

The increasing pressure among pregnant women to avoid excess body fat and lose weight very quickly after giving birth has been attributed to media coverage of celebrities such as Victoria Beckham, Catherine Zeta-Jones and Madonna who regained their pre-pregnancy figures within weeks of giving birth.

Previous research carried out by scientists at the University of Southampton showed that children whose mothers are deficient in vitamin D during pregnancy may grow up with lower-than-normal bone mass, putting them at risk of osteoporosis-related fractures later in life.

The Canadian study found that each daily cup of milk added, on average, 41g to the birth weight of the baby.

“Although most nutrients in milk may be replaced from other foods or with supplements, vitamin D is found in few commonly consumed foods except for milk,” the report said.

At present, neither the Department of Health nor the Food Standards Agency recommends a specific daily milk dosage. However, all pregnant women should receive a Government advice book from their doctors that emphasises the importance of dairy products as a part of a healthy diet for mother and child. It advises consumption of at least one portion of milk, cheese or yoghurt every day, while expectant mothers on income support can also receive seven pints of cow’s milk free each week.

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