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Children Born in Summer Tend to Be Healthier Adults, Study Suggests; Higher Vitamin D Exposure in Second Trimester May Explain Effect

Children who were born in the summer are more likely to be healthy adults, suggests new research published online on October 12, 2015 in an open-access article in the journal Heliyon, published by Elsevier. The article is titled “Season of Birth Is Associated with Birth Weight, Pubertal Timing, Adult Body Size, and Educational Attainment.” The authors of the study, which involved almost half a million people in the UK, say more sunlight, and therefore higher vitamin D exposure, in the second trimester of pregnancy could explain the effect, but more research is needed. According to the study, birth month affects birth weight and age at menarche, both of which have an impact on overall health in women as adults. The environment in the womb leads to differences in early life, including before birth, that can influence health in later life. The scientists said that their findings provide support for the “fetal programming” hypothesis, refining and extending the impact that season of birth has on childhood growth and development. The researchers behind the new study, from the Medical Research Council (MRC) Epidemiology Unit, University of Cambridge, UK, looked at whether birth month had an effect on birth weight, age at menarche, adult height, and body mass index (BMI). They found that children who were born in the summer were slightly heavier at birth and taller as adults. For women, those born in summer months reached menarche slightly later than those born in winter months. No significant association with summer birth with BMI was observed. "When you were conceived and born occurs largely 'at random' - it's not affected by social class, your parents' ages, or their health - so looking for patterns with birth month is a powerful study design to identify influences of the environment before birth," said Dr. John Perry, lead author of the study. Previous studies have reported certain effects of the season of birth, for example on birth weight and various other health outcomes. Dr. Perry and the team thought that childhood growth and development, including the timing of puberty, is an important link between early life and later health, so decided to study more closely the impact of birth month.

The researchers compared the growth and development of approximately 450,000 men and women from the UK Biobank study, a major national health resource that provides data on UK volunteers to shed light on the development of diseases.

The results reveal that babies, of both sexes, born in June, July, and August were heavier at birth and taller as adults. Females born in these summer months women, reached menarche slightly later than women born in oither months. Concordantly, those born in winter (December–January–February) showed directionally opposite differences in these outcomes.

This is believed to be the first time that age at menarch has been associated season of birth. Later onset of puberty has previously been associated with better health in adult life.

"This is the first time puberty timing has been robustly linked to seasonality," said Dr. Perry. "We were surprised, and pleased, to see how similar the patterns were on birth weight and puberty timing.”

“Our results show that birth month has a measurable effect on development and health, but more work is needed to understand the mechanisms behind this effect."

Additional associations were observed with educational attainment; individuals born in autumn versus summer were more likely to continue in education after age 16 years (P = 1.1 × 10−91) or attain a degree-level qualification (P = 4 × 10−7). However, unlike other outcomes, an abrupt difference was seen between those born in August versus September, which flank the start of the school year.

The researchers believe that the differences between babies born in the summer and the winter months might simply come down to how much sunlight the mother gets during pregnancy, because that, in part, determines her vitamin D exposure, and the baby’s in utero exposure to vitamin D.

"We don't know the mechanisms that cause these season of birth patterns on birth weight, height, and puberty timing," said Dr. Perry. "We need to understand these mechanisms before our findings can be translated into health benefits.”
“We think that vitamin D exposure is important and our findings will hopefully encourage other research on the long-term effects of early life vitamin D on puberty timing and health."

The researchers said “In summary, we provide robust evidence linking season of birth to childhood growth and development, in addition to confirming the known associations of timing of birth and educational attainment. While the associations between season of birth, or estimated ante-natal sunshine exposure, with birth weight are consistent with experimental effects of in utero vitamin D exposure on fetal growth, differing patterns of seasonality and independent associations suggest that other mechanisms may link season of birth to adult height and also puberty timing in women. Future work should aim to better understand the mechanisms linking in utero exposures to outcomes years later in life.”

[Press release] [Heliyon article]

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