Pregnant woman who get in too little vitamin D are more than twice as likely to have a child that develops autism, according to a study by Erasmus MC in Rotterdam among over 4 thousand 6-year-olds and their mothers.

Head researcher and psychiatry professor Henning Temeier made use of Generation R - a large study in which the growth and development of children in Rotterdam are followed - for his study. He looked at the mothers' blood tests at 20 weeks pregnancy and the umbilical cord after birth. He found that the mothers of the 80 children with an autistic disorder all had the biggest vitamin D deficiency during pregnancy.

Also in the milder forms of autism, including inability to play with other children or endless repetition of certain behaviors, there was a vitamin D deficiency in the mother. 

The Health Council advises pregnant women to take 10 micrograms of vitamin D, especially for ensuring strong bones in the baby. But not all mothers-to-be do so. Sunlight helps with the natural production of vitamin D, but might not be enough - especially for people who do not get outside much or have darker skin.