Writing in the American Journal of Epidemiology, the research team found that 8.9% of children conceived between January and March had learning disabilities. This compared to 7.6% of children conceived between July and September.
The team from the Universities of Glasgow and Cambridge point toward the lack of sunlight in the UK during the first three months of the year as the most plausible explanation.
Sunlight is a primary source of vitamin D and during these months sunlight doesn't contain enough UVB radiation for the skin to make vitamin D.
Here vitamin D is primarily obtained from food sources including fortified foods and supplements. Pregnant and breastfeeding women in particular are advised to take a daily 10 μg vitamin D supplement.
Lead by Professor Jill Pell, director at the University of Glasgow's Institute of Health and Wellbeing, the team looked at 801,592 children attending Scottish schools between 2006 and 2011.
Children’s birth months were recorded and correlated to the educational needs of the children.
The number of children that received extra assistance in school was at its highest among children conceived in the first quarter of January to March and lowest in the third quarter of July to September at 8.9% and 7.6%, respectively.
Seasonal variations also remained a significant factor to other neurological conditions such as autistic spectrum disorder, intellectual disabilities and learning difficulties such as dyslexia.
However, the team found the month of birth had no bearing on sensory or motor/physical impairments and mental, physical or communication problems.
In total, seasonal variations accounted for 11.4% of all cases.
“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,” said Professor Gordon Smith, head of the Department of Obstetrics and Gynaecology at the Cambridge University and co-author of the study.
“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."
The first three months of pregnancy are considered a crucial period in the brain growth of the foetus. Previous studies have shown that a lack of vitamin D can impair this neurological development.
Other pieces of research have been more cautious in their findings, unable to determine the clinical significance of increasing vitamin D intake during pregnancy.
One review found supplementing pregnant women with vitamin D may reduce the risk of pre-eclampsia, low birth weight and preterm birth. However, when vitamin D and calcium were combined, the risk of preterm birth was increased.
“Our findings are consistent with vitamin D playing a role,” said Pell.
“We found that the first 10 weeks of pregnancy may actually be too late to start supplementation. Women may actually need to start taking it as early as they can and ideally before they even conceive."
Vitamin D’s mechanism of action during pregnancy is complex, with uterine cells and the placenta converting the vitamin into various forms.
Research suggests vitamin D aids implantation and maintains normal pregnancy, supports foetal growth through delivery of calcium, controls secretion of multiple placental hormones and limits production of proinflammatory cells.
New NICE guidance
In 2012, the UK Department of Health's chief medical officer suggested that pregnant women should take vitamin D as a supplement. These recommendations were based on associations between vitamin D and other conditions such as rickets.
This was closely followed by guidance issued by the National Institute of Clinical Excellence (NICE) in 2014, which recommended better supplement availability, and more awareness of low vitamin D status for pregnant and breastfeeding women.
“The NICE guidance says that clearer recommendations are needed from all organisations involved in tackling low vitamin D levels. For example, the uptake of Healthy Start supplements among pregnant and breastfeeding women and young children is reported to be less than 10%,” said Professor Susan Jebb, chair of the independent committee which developed the NICE guidance.
Last month saw further recommendations by the UK government that advised that adults and children over the age of one to get 10 micrograms (μg) of vitamin D every day.
The Reference Nutrient Intake (RNI) of 10 µg/d for the general UK population included pregnant and lactating women and population groups at increased risk of vitamin D deficiency and should come from a combination of sunlight, naturally occurring dietary sources and fortified foods and supplements.