Study: Soaking Up Rays May Produce Bigger Babies

Ultraviolet sunlight exposure in the third trimester of pregnancy may be related to increased height and bone health of children.
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After years of hearing it can cause first-degree burns, skin cancer and accelerated aging, some good news about ultraviolet sunlight has emerged: Increased height and bone health of children is related to the amount of sunlight mothers receive while pregnant.

According to research published in the March edition of the Journal of Clinical Endocrinology and Metabolism, children born in the late summer and early fall are taller and have higher bone density and mineral content than their counterparts because their mothers are exposed to greater levels of UVB rays during the third trimester.

To determine the relationship between ambient maternal sunlight exposure and child bone health, Adrian Sayers and Jonathan Tobias, two University of Bristol rheumatology researchers, first consulted the medical records of nearly 7,000 children participating in the Avon Longitudinal Study of Parents and Children. ALSPAC is an ongoing long-term health research project that monitors the health and development of 14,000 children born to English mothers between April 1991 and December 1992. Then, Sayers and Tobias used historical meteorological records for the region around Bristol and Bath to calculate total environmental UVB levels each mother experienced during the 98 days prior to giving birth.

Statistical analysis of their database revealed children born after third trimesters with high total UVB levels (i.e. births between June and October) had longer birth lengths and greater height and weight than those born during the winter and early spring. For every four weeks of "summer" sunlight a mother had above the average, the height and weight of the child at 10 years of age increased by 0.18 centimeters and 0.1 pounds respectively. (The study did not track how much any individual spent in the sun for those summer months.)

The strongest association, however, was between UVB exposure and greater bone area, density and mineral content when the child was 10 years old. Four weeks of summer sun exposure was associated with increases of 8.1 cm2 in bone area, 0.003 g/cm2 in bone density and 9.6 gram in bone mineral content. While small, these results indicate elevated sunlight exposure reduces fracture risk of the child by approximately 5 percent (normally the association is reversed and fracture risk increases in taller individuals).

Sayers and Tobias believe the positive trend height, weight and bone characteristics are due to greater levels of vitamin D in the mother during the third trimesters when critical, early skeletal development occurs in the womb (UVB rays play a key role in generating vitamin D in the body). Their findings are consistent with other studies focusing on in utero vitamin D levels and offspring health, but this study was the first to include enough participants to create statistically significant results. Past studies have reported increased vitamin D is not only associated with greater bone mineral content but also the lowered risk of Type 1 diabetes and asthma.

While many women may be looking for a good reason to sunbathe, Sayers and Tobias emphasize that the results of this study are not suggesting mothers should risk sun damage for possible improved health of their unborn child. Instead, Tobias was quoted in a press release, "Pregnant women might consider talking to their doctor about taking vitamin D supplements, particularly if their babies are due between November and May, when sunlight levels are low."

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