Women who do not get enough vitamin D in pregnancy may have children with poorer social and motor skills, according to new research.
The NHS recommends all pregnant and breastfeeding women take a daily supplement of vitamin D.
But a new study from the universities of Surrey and Bristol found a lack of vitamin D in pregnancy has a negative effect on the social development and motor skills of pre-school age children.
Publishing in the British Journal of Nutrition, the team examined data from more than 7,000 mothers and their children.
They found that pregnant women who were deficient in vitamin D (less than 50 nmol per litre in blood) were more likely to have children with low scores (in the bottom 25%) in pre-school development tests than those whose mothers got enough.
The tests examined gross and fine motor development at age two-and-a-half.
The tests included kicking a ball, balancing and jumping and using fine motor skills to hold a pencil and build a tower from bricks.
Those women who did not get enough vitamin D in pregnancy also had children whose social development was regarded as poorer at age three-and-a-half.
However, there was no link between vitamin D intake and some skills at an older age, such as IQ and reading ability at ages seven and nine.
Sunlight is the main source of vitamin D for most people, but experts are unable to say how much sun is the "right amount", owing to differences in how people's bodies convert vitamin D.
The vitamin is also found naturally in a small number of foods such as oily fish (including mackerel and fresh salmon and tuna), red meat, liver and egg yolks, and is added to some breakfast cereals and spreads.
Experts believe vitamin D and dopamine, a neurotransmitter released by the brain, interact during foetal development, playing a crucial role in the neurological development of brain areas controlling motor and social development.
Lead author Dr Andrea Darling, from the University of Surrey, said: "The importance of vitamin D sufficiency should not be underestimated.
"It is well-known to be good for our musculoskeletal systems, but our research shows that if levels are low in expectant mothers, it can affect the development of their children in their early years of life."
She said although vitamin D is found in food, unless a large portion of oily fish (100g) is eaten every day "it is difficult to get the recommended daily intake of 10 micrograms per day from food alone".
She added: "Many pregnant women, especially those from minority groups with darker skin (eg African, African-Caribbean or South Asian), will still need to take a 10 micrograms vitamin D supplement daily, particularly in the autumn and winter when vitamin D cannot be made from the sun in the UK.
"However, it is important to remember that 'more is not necessarily better' and it is important not to take too much vitamin D from supplements as it can be toxic in very high doses."