AUTISM, dyslexia and other learning disabilities are more common in children conceived in the first three months of the year, according to new research.

A study of more than 800,000 Scottish youngsters found the conditions were more prevalent in those conceived between January and March.

Scientists from Glasgow and Cambridge universities suggest the finding may be linked to vitamin D deficiency in mothers.

Professor Jill Pell, director of Glasgow’s Institute of Health and Wellbeing, added: “It is important that pregnant women follow the advice to take vitamin D supplements and also that they start supplements as early in pregnancy as possible – ideally when they are trying to get pregnant.”

The study, which also involved the NHS and the Scottish Government, found seasonal differences related to autism, intellectual difficulties and learning difficulties such as dyslexia.

Researchers found 8.9 per cent of those conceived in the first quarter of the year had learning disabilities, compared with 7.6 per cent of those conceived between July and September.

However, there were no patterns for other causes of learning difficulties such as visual or hearing problems, or physical illness.

All children in the research attended Scottish schools between 2006 and 2011 – and so were born before 2012 guidelines advising pregnant women to take vitamin D supplements to prevent other conditions, such as rickets, were drawn up.

The data came from the annual pupil census, which covers both primary and secondary levels and includes mainstream and special schools. According to the 2011 Scottish census, 99 per cent of children aged five to16 with learning disabilities are in some form of education.

The study also suggested exposure to maternal infection could be a factor, with the January-March period also linked to peaks in seasonal flu.

Most previous work has focused on the month of birth, but researchers said focusing on conception avoided variations in gestation to improve analysis.

However, Professor Gordon Smith, department head of obstetrics and gynaecology at Cambridge, said: “If vitamin D levels do indeed explain the seasonal fluctuations observed in this study, we would hope that widespread compliance with the advice would lead to loss of this variation, and would have a downward effect on overall rates of special educational needs.

“Although the current study did not directly measure vitamin D, it remains perhaps the most plausible explanation for the trend.

“Hence, these findings underline the importance of health professionals recommending vitamin D, and the importance of women complying with the treatment to optimise their chances of a healthy child.”

Researchers wrote that the season of conception is “strikingly associated” with risk of special educational needs, stating: “Our study was large and non-selective, including children in public schools across the whole of Scotland. The pupil census does not include private schools but, in Scotland, fewer than five per cent of children attend private schools.

“We were able to examine a range of causes of special educational needs and therefore to explore whether seasonal patterning was specific to one or more causes.”

They went on: “As seasonal variability accounts for a substantial health economic burden of disease and biologically plausible causes of this variation are potentially amendable to intervention, these observations are potentially highly relevant for public health.”

The research is published in the American Journal of Epidemiology.