Low vitamin D intake during pregnancy is associated with lower birth weights in babies, according to a study.
The research compared babies' birth weights with the amount of vitamin D fortified milk and vitamin D supplements women took while pregnant.
The researchers from McGill University, Canada, suggest the study shows vitamin D may be an important regulator for foetal growth.
The study is published in the Canadian Medical Association Journal.
The study involved over 300 pregnant women, based in Canada, who were asked to fill in questionnaires about their diet and lifestyle. This included information about how much milk they consumed and the dietary supplements taken.
Milk contains protein, riboflavins and calcium, but in Canada it is fortified with vitamin D.
Sunlight is the normal source for vitamin D, but many people have vitamin D deficiencies, and it is difficult to get from common food sources.
The researchers discovered that women who drank less than 250ml of fortified milk a day had lower intakes of vitamin D and protein than women who drank more than this amount.
After recording the birth weights of the infants, the researchers found that women who drank less milk and had lower intakes of vitamin D were more likely to give birth to smaller babies.
They discovered that as the intake of fortified milk increased, birth weight went up, and for every extra 250ml drunk a day, there was an increase of 41g in birth weight.
An increase of one microgram of vitamin D per day was linked to an 11g rise in birth weight.
The level of protein, riboflavin and calcium were not found to be linked to the weight of the babies.
The authors of the paper are worried that increasingly women choose, or are advised to, restrict milk during pregnancy. They do this for a number of reasons, including lactose intolerance or wanting to prevent childhood allergies.
But the scientists believe milk and vitamin D are important sources of nutrition during pregnancy, and say the study shows their importance for childhood development.
In an accompanying commentary in the journal, Professor Bruce Hollis of the Medical University of South Carolina, US, highlighted the importance of vitamin D, and said it may be linked to neurodevelopment, immune function and chronic disease susceptibility in developing babies, as well as birth weight.
"This paper is important because it shows the benefit of very small amounts of vitamin D," he said.
"If we could get this level to meaningful amounts, the effects could be dramatic."
Dr Gail Goldberg, a nutritionist from the Medical Research Council' Human Nutrition Research Centre, Cambridge, UK, said: "This is an interesting study, and very topical as there is a lot of interest in this area.
"In Canada, milk is fortified, and a lot of the vitamin D intake seems to come from this source."
"In other countries, sunlight or supplements are the main source as it is not found in common foods, but there is a lot of dispute between scientists as to how much is the optimal amount to take a day."