Pregnancy and child

Sunlight and young bones

“Women who are pregnant during the summer have taller, stronger-boned babies” reports The Independent, describing research on 7,000 children as part of an 18-year study. According to the newspaper, a mother’s exposure to the sun’s “vitamin-boosting rays”, can give her child larger, healthier bones. Exposure to the sun triggers the body to produce its own vitamin D, although it is also available through diet and taking vitamin supplements.

The research was conducted by estimating pregnant mothers’ exposure to the sun using weather records, and comparing full-body bone scans of their children around the age of 10. While the newspaper focuses on children’s height, the researchers emphasise their findings that UVB exposure affects bone mass, primarily increasing the width of bones.

While sunlight can increase the body’s vitamin D levels there is established evidence that sunbathing and exposure to high levels of the sun’s ultraviolet light are a health risk. Current guidance from the National Institute for Health and Clinical Excellence (NICE) states that it is important to maintain adequate vitamin D during pregnancy and breastfeeding, and that expectant mothers may choose to take up to 10 micrograms of vitamin D a day through supplements.

Where did the story come from?

Dr Adrian Sayers and Jonathan Tobias from the University of Bristol carried out this study. Their work was funded by the UK Medical Research Council, the Wellcome Trust and the University of Bristol. It was published in the Journal of Clinical Endocrinology & Metabolism, a peer-reviewed medical journal. 

What kind of scientific study was this?

This research was a cohort study exploring the relationship between pregnant women’s vitamin D exposure and the bone development of their offspring at around 10 years of age. Several previous studies suggest that a mother’s exposure to vitamin D in pregnancy affects her child’s bone development.

In this new study the researchers focused on whether exposure to UVB rays in sunlight during the third trimester of pregnancy was related to a child’s bone mineral content (BMC), a measure of bone mass. They also wanted to explore whether this relationship was due to the effects of vitamin D levels on height, fat or lean mass (muscle weight), or whether BMC was independent of these factors.

The researchers used data from the Avon Longitudinal Study of Parents and Children (ALSPAC), a prospective cohort study set up to investigate what factors influence the health, growth and development of children. It followed children born to all pregnant women living in the Avon region of South West England predicted to give birth between April 1991 and December 1992.

About 14,000 women were enrolled in the ALSPAC study, but this publication only considers the 6,995 of their children that had both bone scans and data available on their mothers’ UVB exposure. Whole-body bone scans were performed when the children were around the age of nine years.

All of the data in the ALSPAC study was collected through postal questionnaires, computer records, physical examination of the children and by reviewing medical records. Mothers’ exposure to sunlight was estimated by considering which months their third trimester of pregnancy fell in, and relating these to meteorological records. In this way the researchers could work out how much UVB mothers would have potentially been exposed to in the 98 days before the birth of their child.

The level of vitamin D in the blood (serum total 25-hydroxyvitamin D) was also measured in a subgroup of participating mothers (355 of them) when they were an average of 36 weeks pregnant.

Researchers used statistical methods to explore the effects of UVB exposure and BMC, looking at length at birth plus bone density, BMC, weight and height at an average age of 9.9 years.

They also further explored the link between exposure to UVB and bone area by analysing effects on longitudinal bone growth and periosteal bone growth (thickening) separately.

What were the results of the study?

 The study found that exposure to UVB was related to BMC, bone mineral density (BMD) and to bone area.

The researchers say that when they took into account the effects of height and of muscle mass, there was still a positive link between UVB exposure and bone area of the children at 9.9 years of age. They say that “although maternal UVB exposure was associated with height, the strength of the relationship was somewhat weaker than that with bone area”. They say that this is due to equal effects of UVB exposure on bone lengthening and thickening.

The researchers say that presuming the effects on bone mass persist to adulthood, they represent a reduction in fracture risk for these participants.

What interpretations did the researchers draw from these results?

The researchers conclude that their results suggest that maternal vitamin D exposure influences later skeletal development of children by affecting bone size.

They say that it is interesting that the link between UVB exposure and bone area was stronger than that with height. This, they say, could be because UVB exposure affects periosteal bone growth (widening of bones) rather than lengthening of bones.

What does the NHS Knowledge Service make of this study?

This study adds to what is already known about the benefits of vitamin D during pregnancy, that UVB affects bone size, but not just its length. However, while some news sources suggest that pregnant women may even consider sunbathing, the risks of exposing skin to high levels of UV light are well established. Most people can get adequate sunlight exposure to produce vitamin D through normal amounts of time spent outdoors without needing to sunbathe.

Also, some media reports have emphasised a link between UVB exposure and height in children, saying, for example, that “summer babies are tall and strong”. Other studies have suggested there may be a link between sunlight in pregnancy and children’s height, but the main findings from this new research are actually concerned with bone width.

Points to note when interpreting this study:

  • The researchers found that UVB exposure is linked to bone mass, to both bone length and to other dimensions of bone growth such as thickening. This suggests that maternal vitamin D exposure influences the skeletal development of the children primarily by affecting bone size. This link to bone area is a stronger link than to height.
  • The researchers found that the bones of children whose mothers were exposed to more sunlight were wider than other children. This was to a degree greater than expected, even in relation to the children’s additional height.
  • It is unclear whether the researchers adjusted for other factors that may be linked to bone mass in the children. Most importantly, this would be sunlight exposure throughout a child’s life, and lifestyle factors including diet.
  • The researchers also say that because fat, bone mass and lean (muscle) mass are so closely related, it’s not possible to conclude absolutely whether the primary effects of UVB exposure were on bone mass or on muscle mass.
  • The researchers caution that because of the two-dimensional scanning methods they used in their study, confirmation is needed to determine exactly which bone dimensions are being affected by UVB exposure.

Vitamin D is important during pregnancy and current NICE guidance says that it is important to maintain adequate vitamin D during pregnancy and breastfeeding, and that women may choose to take up to 10 micrograms of vitamin D a day during these periods.

Supplements are not routinely given to pregnant women, but the NHS does provide supplements containing vitamin D for children aged between six months and four years.

NHS Attribution