Changing Trends in Birth Seasonality in Canada

It often comes as a surprise that there exists a strong seasonal pattern in births. Historically, the seasonal pattern in births has been linked to factors such as nutrition, urbanization, and climate while modern patterns in births are more driven by choice, made possible through various forms of contraception. However, it is also possible for that choice to be influenced by public policy. This trends blog post examines the link between public policy and changes to birth seasonality in Canada.  

Historical trends in birth seasonality in Canada have been well documented. Cowgill (1966); Halli (1989); Trovato and Odynak (1993); Werschler and Halli (1992) use data on births in Canada ranging from 1926-1989 to show that prior to 1990, births in Canada had a spring (April to June) peak. Has this seasonal pattern continued on since then?

The figure presents standardized quarterly birth data for Canada using what is known as a heat map. A heat map is a data visualization tool where values are depicted by colour. In this case, birth seasonality is being depicted where the yellow colour is a quarter with fewer standardized births than other quarters. The darker the colour, the more standardized births take place in that quarter. The year of birth is on the vertical axis in ascending order and the birth quarters are along the horizontal axis.

The figure confirms the historical pattern of births up to 2001. As found in the historical literature, between 1980 and 2000 in births in Canada displayed a strong seasonal pattern with the spring quarter being the peak season for births. However, the spring birth pattern changed around 2001. In 2001 spring births actually began a permanently shift to the summer. That is, the seasonal pattern of birth in Canada changed in 2001, shifting from spring to summer. We also see that standardized births fell in winter and rose in the fall at the same time, indicating a shift of standarized births from the first half of the year to the second half of the year.

Data Source: Author Calculatons using Sigtstice Caneds Tabe 13-10-0415-01 (1191-2020) & Vital Statistes Pubicaton (1972-1990)

What happened around 2001 that could have changed birth seasonality in Canada so dramatically? In 2001, paid parental benefits were significantly expanded from 10 weeks to 35 weeks adding to the already existing 15 weeks of paid maternity leave. By 2001, 70-75 percent of mothers qualified for maternity and parental leave benefits (Marshall 2010). 

Since there is no universal explanation for birth seasonality in the first place, it is  challenging to posit whether the trends we present here are behavioural responses to the parental leave policies, or if the shifts in seasonality are due to some other factors unrelated to policy timing. There does exist plenty of evidence that shows that delivery dates are influenced by public policy (See Compton and Tedds 2016 for a detailed discussion). Compton and Tedds (2016) show that no similar changes to birth seasonality occurred during this time in Northern U.S. states and are able to isolate the change in birth seasonality in Canada to married women between the ages of 25-34 (those most likely to be planning a pregnancy). These pieces of evidence suggest that it is possible that changes in the duration of parental leave changed the preference related to season of birth to better align births and annual vacations, but there is, as of yet, insufficient evidence to prove this causally. 

Why should we care about birth seasonality? First, there is well documented negative health effects for those born in spring months in the northern hemisphere, including higher incidence of multiple sclerosis and schizophrenia. A shift away from spring births may mean lower incidence of these diseases. Second, children born in the spring months in the northern hemisphere also have higher birth weights than children born in other seasons. In fact, correlated with the change in birth seasonality has been a drop in birth weights, from 3442 grams to 3367 grams (Adam et al. 2022). Since children with higher births weights are more likely to develop childhood diabetes this may translate into a lower incidence of this disease (Adam et al. 2022). Third, as many provinces use the calendar-year for school entry, more children born later in the year means more younger students in the classroom which may affect school readiness and test scores. These effects will be more noticeable in jurisdictions that have junior kindergarten. Finally, it also means that pre-school providers will experience changes in the timing for peak enrollment and may have to plan accordingly.   

Since the start of the COVID-19 pandemic, there has been much discussion about if and how the pandemic and pandemic supports may have affected births in Canada. At this point in time, there is insufficient data available to study the effect on either overall births or the season of birth. However, this discussion shows that studying these issues will be important as more data becomes available.  

Dr. Lindsay M. Tedds is an Associate Professor in the Department of Economics at the University of Calgary. She can be contacted at lindsay.tedds1@ucalgary.ca. The author declares they have no competing interests, either financial or community in nature. 

REFERENCES 

Adam, Shiraz El, Jennifer A. Hutcheon, Chris McLean, and Kim McGrail. 2022. Why are babies in Canada getting smaller? https://www150.statcan.gc.ca/n1/pub/82-003-x/2022001/article/00001-eng.htm. 

Compton, Janice, and Lindsay M. Tedds. 2016. "Effects of the 2001 Extension of Paid Parental Leave Provisions on Birth Seasonality in Canada." Canadian public policy 42 (1): 65-82. https://doi.org/10.3138/cpp.2015-054. 

Cowgill, Ursula M. 1966. "The Season of Birth in Man." Man (London) 1 (2): 232-240. https://doi.org/10.2307/2796349. 

Halli, S. S. 1989. "The seasonality of births in Canada." Journal of biosocial science 21 (3): 321-327. https://doi.org/10.1017/S0021932000018010. 

Lam, D. A., and J. A. Miron. 1996. "The Effects of Temperature on Human Fertility." Demography 33 (3): 291-305. https://doi.org/10.2307/2061762. 

Lam, David A., and Jeffrey A. Miron. 1994. "Global Patterns of Seasonal Variation in Human Fertility." Annals of the New York Academy of Sciences 709 (1): 9-28. https://doi.org/10.1111/j.1749-6632.1994.tb30385.x. 

Marshall, Katherine. 2010. "Employer top-ups." Perspectives on labour and income 22 (1): 43. 

Trovato, Frank, and Dave Odynak. 1993. "The Seasonality of Births in Canada and the Provinces, 1881-1989: Theory and Analysis." Canadian studies in population 20 (1): 1. https://doi.org/10.25336/P6CS3G. 

Werschler, Timothy, and Shiva Halli. 1992. "The Seasonality of Births in Canada: A Comparison with the Northern United States." Population and environment 14 (1): 85-94. https://doi.org/10.1007/BF01254608.