Babies born in the summer are more likely to be heavier at birth, taller as adults and healthier overall, a study at Cambridge University has found.
Researchers at the Medical Research Council (MRC) Epidemiology Unit concluded that pregnant mothers getting more time in the sunshine may be responsible for the differences but said further tests were needed.
More than half a million people were involved in the study, published in the journal Heliyon, which studied the growth and development of UK men and women.
Dr John Perry, the lead author and a senior investigating scientist, said: “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.”
Previous studies had reported the effects of birth season on weight and health, leading Dr Perry and the team to study the timing of puberty, an important link between early life and later health, more closely.
Their results revealed that babies born in June, July and August were heavier at birth and taller as adults.
The study also revealed that girls born in the summer started puberty later – an indication of 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.”
Babies born in the coldest winter months of December, January and February tended to be lighter at birth, shorter at adults and go through puberty earlier.
Researchers concluded that the environment in the womb leads to differences in early life, including before birth, and development throughout childhood and into adulthood.
They suggested that higher vitamin D exposure in the second trimester of pregnancy could explain the effect.
“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."
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