Girls Born in the Summer End Up Healthier Adults

A new study finds kids born in summer months tend to be healthier adults, a phenomenon linked to a mother's exposure to sunshine during pregnancy. REUTERS/Regis Duvignau

A girl's timing of puberty and her overall health as an adult may be partially determined by the month in which she was born, according to a new study published Monday in the journal Heliyon.

The study was based on data collected on 450,000 men and women as part of the ongoing United Kingdom Biobank project, a database that tracks health and disease trends in the country. The paper suggests that an expecting mother's exposure to sunlight during the second trimester of a pregnancy may be critical to the development of a fetus. Sun exposure helps the body produce vitamin D, an essential building block for good health.

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A team of University of Cambridge researchers found that females born in the summer months were heavier at birth than those born in the wintertime. They were also taller as adults and had a later onset of puberty. Those born in summer months were also more likely to attain a higher level of education. The researchers also looked at data on body mass index, but they didn't observe any marked differences between people born in the summer and winter.

"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," John R.B. Perry, lead author of the study, said in a press statement.

A handful of studies have shown that a vitamin D deficiency in pregnant women can cause a host of developmental problems. One study published in Pediatrics involving nearly 1,000 women found vitamin D deficiency during pregnancy was associated with impaired lung development in 6-year-old offspring, neurocognitive difficulties at age 10, increased risk of eating disorders in adolescence, as well as lower bone mass at age 20.

This is also not the first study to observe that birth season influences adult health outcome. A study published in 2012 found higher rates of autoimmune diseases among people born in winter months. Another study linked spring births to higher rates of type 1 diabetes. Birth month also appears to impact risk for psychiatric illness and cardiovascular disease.

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All of this research sheds light on a phenomenon called the "fetal programming hypothesis," which is the idea that the environment of the womb influences a baby's development and this development impacts the person's health throughout their lifetime. Factors that contribute to fetal programming include environmental toxins and pollution, nutrition and hormone changes—such as fluctuations in the stress hormone cortisol. But the authors of this current study say more research needs to be conducted to understand the mechanisms at work behind differences in health based on birth month. It is possible there may be other factors at play beyond vitamin D exposure from extra sunshine.

"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," said Perry.