Children who are born during summer are more likely to be heavier at birth and taller as adults than those born in the cooler months, according to a study that followed the development of 450,000 people in the UK.
For women, there seem to be even more benefits, with the study finding that summer baby girls actually start their periods later than those born during the rest of the year - which is an indicator of female adult health.
To be clear, this was an observational study and it only found a correlation between these health markers and the timing of birth, so we still have no idea what could be causing the link. But the researchers suggest that getting more sun - and therefore higher vitamin D exposure - in the second trimester of pregnancy might explain the effect.
"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," said lead researcher John Perry, from the Medical Research Council in the UK.
It's well established that the environment babies experience in the womb can lead to differences in health later in life - an effect known as programming.
Previous studies have also found a link between the season a child is born and their risk of developing certain diseases. But this research was the first to look specifically at birth weight, adult height, and puberty timing, and it's the first to link the time a female gets her first period to the seasonality of her birth.
"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 Perry.
To understand this relationship, the researchers analysed data on more than 450,000 men and women taken from the UK Biobank study, and looked at how birth timing affected their health during development and into adulthood.
They found that not only were summer babies heavier at birth, taller as adults, and - in the case of females - later to start puberty than those born the rest of the year, they also showed that babies born during the winter months followed the opposite trend - they were shorter as adults, lighter at birth, and the females got their periods earlier.
Publishing in the journal Heliyon, the researchers explain that the same links were also seen when they looked at total hours of sunshine during the second trimester, but not during the first tree months after birth, which has led them to believe that it's in utero vitamin D exposure that may be driving the difference.
The authors hope that the study will encourage further research into this link, and one day potentially reveal ways that women can help improve the health of their baby, regardless of when they conceive.
"We don't know the mechanisms that cause these season of birth patterns on birth weight, height, and puberty timing," explained Perry. "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."
Oh, and just in case you were wondering, here's a list of the conditions you're most likely to develop based on your birth month if you're in the Northern Hemisphere. For those of us on the other side of the world, the associations are the opposite.
