Why are babies in Canada getting smaller? Several factors may play a role, report says

An increase in the number of small-for-gestational-age births—an important indicator of infant health—has been observed in data that goes back more than 20 years in Canada.

And while the reasons for this increase remain largely unclear, a University of British Columbia (UBC) report hypothesizes that systemic issues such as immigration, economic stress and increasing maternal age may play a role.

Using Statistics Canada data that tracked trends in birth weight and small-for-gestational-age births, or SGAs, births among all single-baby live births in Canada from 2000 to 2016, the researchers found the mean birth weight for all births decreased from 3,442 grams in 2000 to 3,367 grams in 2016.

During the same time period, the rate of SGA births increased from 7.2 per cent in 2000 to eight per cent in 2016, corresponding with a 12 per cent increase in the odds of having a smaller baby in 2016 compared to 2000.

“Historically in Canada, we knew that babies were actually getting bigger, specifically between 1978 and 1996. That trend, at the time, was explained by sociodemographic factors and also by changes in maternal body size and maternal smoking,” study author Shiraz El Adam told CTVNews.ca by phone Wednesday.

“We used the same rationale and looked at population-level factors that have been changing over time in Canada that we know are also linked to a higher risk of a smaller baby.”

The study took into consideration sociodemographic factors previously associated with lower birth weight, including the birthplace of the baby’s mother and father.

In some countries, particularly in Asia, healthy babies are naturally smaller than they are in North America. With the increase in immigration rates in Canada, researchers hypothesized that immigration might explain the domestic decline in birth weight.

Results from the modelling showed that the odds of an SGA birth were indeed higher among births to parents born outside of Canada—but that only explained part of the decline.

The odds of having a smaller baby were also higher for unmarried women, older women and those who lived in lower-income neighbourhoods. Yet still, when researchers adjusted for these factors, including changes in neighbourhood income to take into consideration the material environment that may impact expectant mothers, the trend couldn’t be fully explained.

“Despite accounting for the factors that we believe to be strongly correlated with the trend, we’re still seeing this unexplained increase. And it appears to be consistent also with other countries, for example, the U.S. and in Germany and in Japan, and more recently in Italy,” said El Adam.

“They have tried to explain it as well in the U.S. and in Germany using similar factors and it also remains unexplained.”

While the study doesn’t draw a specific correlation between economic stressors and SGA births, the report notes that several other studies have looked at the negative financial hardship and stress can have on expectant mothers.

The World Health Organization also notes that adverse birth outcomes may be connected to larger macroeconomic factors, such as an economic recession.

The study notes that the findings show only a modest reduction in the mean birth weight of babies born in Canada, which may suggest that healthy babies are simply getting smaller over time—a factor researchers plan to evaluate in the next iteration of the study.

“[Birth weight] is a really good indicator for public health, and it’s been monitored for years as an indicator. It’s usually used as a proxy for a growth restriction, but in some cases small babies are small but healthy,” El Adam explained.

“So, I think at this point, the next step would be really to clarify is what’s driving this trend small but healthy babies or a growth restriction. If it is a  growth restriction, then it is really important to identify that because we know fetal growth restriction has not only short but long-term health and economic consequences.”

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