"Vitamin D is important for calcium metabolism, as well as bone growth and development. This has led to a widely held hypothesis that if a mother is exposed to sunlight during pregnancy and receives sufficient UVB radiation, her body produces more vitamin D," said Peeter Hõrak, professor of behavioral ecology at the University of Tartu. Similarly, folate — found in abundance in fresh fruits and vegetables harvested in summer — is vital for fetal and child development.

Hõrak and his colleagues had the opportunity to assess the seasonal impact on development using local data. They examined a large dataset of Estonian children's body measurements originally collected by anthropologist Juhan Aul and later digitized by Markus Valge. The research team analyzed data from more than 20,000 children. The children, aged 7 to 19, were born between 1937 and 1962 and the birthdate was known for each of them.

Using the dataset, the team evaluated the effect of birth season on 11 body measurements, including height, weight and body mass index. "We found that hip width, weight and sitting height — which is the body's length without leg length — were most strongly correlated with the season or month of birth," Hõrak explained.

Sunshine or fresh salad

Juhan Aul measured children over several decades. According to Peeter Hõrak, his research team therefore aimed to assess the effect of birth timing on as many traits as possible to determine which parts of the body are most sensitive to the season of birth. Among other factors, they looked at shoulder width, hip width, lung capacity, skull volume and grip strength. "Aul also recorded sitting height, which can be used to calculate leg length. It's well known that leg length is a particularly sensitive indicator of childhood growth conditions," Hõrak added.

The analysis showed that birth season influenced the majority — nine out of eleven — of the traits studied, most notably hip width, weight and sitting height. Hõrak said it was surprising that the time of year a child was born did not appear to affect lung capacity or strength, which are typically good indicators of overall health. "Especially grip strength showed no connection at all to birth month. That was a bit unexpected for us," he noted.

The main dividing line emerged between children born in the spring and summer versus those born in the fall and winter. "Children born in summer were clearly heavier. They had a higher body mass index and chest circumference. As mentioned, they also had greater sitting height and shoulder width, while hip width was particularly sensitive to seasonal variation," Hõrak explained.

According to him, one possible explanation is that the mothers of children born in summer were exposed to more sunlight in the months leading up to childbirth, prompting their bodies to produce more vitamin D. "Another possible explanation is that these mothers consumed a lot of fresh fruits and vegetables in the final stages of pregnancy. Likely, they also had a good nutritional status during the breastfeeding period," the professor suggested. Fruits and leafy green vegetables are rich in folate, which Hõrak noted plays a regulatory role in biochemical processes vital for child development. A third possibility is that there are fewer respiratory infections during the warmer months, meaning fewer maternal illnesses that might disrupt fetal development.

Which of these factors had the greatest impact on the variation in children's body measurements cannot be determined from this study alone, Hõrak said. "We don't have any biochemical data. All we can say is that our findings are consistent with the idea that warm, sunny weather late in pregnancy or shortly after birth is linked to effects that are reflected later in the child's body size," he concluded.

Likewise, there is no scientific consensus on whether vitamin D has a greater effect earlier or later in pregnancy. According to Hõrak, the fetus is just beginning to develop in the first trimester and is thus highly sensitive to external influences. "The alternative explanation is that the third trimester is a period of rapid long bone growth, which affects height. So perhaps that's when the mother is more sensitive to sunlight exposure," he speculated. The findings of his team's study tend to support the hypothesis that sunlight has more impact later in pregnancy.

Economic situation still a factor

Among other aspects, Peeter Hõrak and his colleagues also examined how the effect of birth season on a child might be linked to the mother's socioeconomic background. Previous research had shown that among the mothers of the children studied by Aul, manual laborers had the lowest life expectancy compared to skilled workers and women in non-physical occupations.

According to Hõrak, all of the children in this study grew up under Soviet conditions, where social classes were officially said not to exist. Yet the influence of class on body measurements was clearly evident. "When we looked at the two groups separately, we saw seasonal effects only in children whose mothers were manual laborers," Hõrak pointed out.

In his view, this finding suggests that working-class mothers likely had fewer resources to buffer their children from adverse environmental factors, such as vitamin D or folate deficiency. In contrast, the body measurements of children whose mothers worked in indoor or skilled jobs did not appear to be affected by season of birth. "Here we see how social stratification can influence child growth," the professor noted.

He believes the study remains relevant today. First, Estonia underwent significant social stratification in the early 1990s during the transition to a market economy. Children who grew up during that period are now parents themselves. "We have seen and shown that, at that time, the impact of parents' social class on children's growth was even more pronounced," said Hõrak.

Second, he noted that one might assume the influence of seasons on children's development has diminished in today's era of consumer abundance. "Yet seasonal effects on child growth are still observed in welfare states like Norway, the United Kingdom, Switzerland and Austria. This is not some peculiarity of Soviet-era conditions," he emphasized.

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Peeter Hõrak, Richard Meitern, Markus Valge and Velda Lauringson have published their findings in the Journal of Developmental Origins of Health and Disease.