Workers Health & Safety Centre

Night work and long hours unhealthy for pregnant workers, says research

New research and an updated literature review provide mounting evidence of the link between working at night or long hours and adverse pregnancy outcomes.
 
Danish researchers recently reviewed data from more than 22,000 pregnant women employed primarily in hospitals. They published their results in the Journal Occupational and Environmental Medicine. Data indicates after the eighth week of pregnancy and up to week 22, working two or more night shifts in a week led to a 32 per cent higher risk of miscarriage in the following week, compared to pregnant women who did not work at night the previous week. The data also indicates an increased risk with each additional night shift worked along with consecutive nights.

Though researchers did not determine a definite reason for the link, they cite evidence suggesting a pregnant woman's exposure to light at night can disrupt their body clock (circadian rhythm) and decrease the release of melatonin, thus interfering with maternal and fetal hormones necessary to support a successful pregnancy.
 
A recent literature review conducted by University of Alberta researchers and published in December 2019 in the American Journal of Obstetrics and Gynecology came to similar conclusions. They reviewed related studies from 33 countries involving almost 200,000 women. The collective data revealed pregnant women working steady nights had 23 per cent higher odds of suffering a miscarriage than pregnant women working a fixed day shift. These same night shift workers also had 21 per cent higher risk of preterm delivery. While working rotating shifts increased the odds of preterm delivery by 13 per cent.

The Albertan researchers also found long hours added to risk, with those working more than 40 hours per week experiencing a 38 per cent higher risk of miscarriage and a 21 per cent higher risk of preterm delivery. Those working more than 55.5 hours each week had a 10 per cent increase risk of preterm delivery compared to those working 40 hours each week.
 
The review data also showed various fetal outcomes including 43 per cent higher risk of having a low birthweight infant for those working long hours and 18 per cent increased risk of having a small for gestational age infant when working rotating shifts.

Path to protective reassignment

Of the Albertan study, co-author Chenxi Cai, a University of Alberta post-doctoral fellow, explains, “We wanted to evaluate the impact of shift work and long working hours during pregnancy to help women and employers make more informed decisions when it relates to occupational hours and pregnancy.”
 
Some employers have taken progressive steps implementing workplace policies to ensure protective reassignment. In other cases, women workers and their representatives have turned to collective bargaining to achieve the same. Many though remain with little, if any, path to protection.
 
The Danish researchers suggest their findings may prompt action, concluding “The new knowledge has relevance for working pregnant women as well as their employers, physicians and midwifes. Moreover, the results could have implications for national occupational health regulations.”
 
Here in Canada, limited reassignment protection under occupational health and safety law is in place in Quebec and for workers under federal jurisdiction (Part II and Division VII, Part III, Canada Labour Code).

Other jurisdictions, including Ontario, Manitoba and Saskatchewan, have some protection under human rights provisions. Ontario women, for instance, have a right to request changes to job duties or rules that affect their health when pregnant. Employers must accommodate these requests, unless doing so would cause undue hardship. Although it should be noted, the Human Rights Commission charged with enforcing these provisions look at each case and will determine any action based on its own merit.
 
Without adequate protection, workers, their representatives and other advocates continue to press for more specific occupational health and safety regulatory protections for pregnant women and nursing mothers.
 
As a last resort, leave without pay is available in Ontario and other jurisdictions.

Wider health implications

In addition to the adverse pregnancy outcomes discussed above, working nights may be responsible for 180 to 460 new cases of breast cancer here in Ontario each year.
 
Furthermore, the most recent evaluation by the International Agency for Research on Cancer (2019), re-confirmed classification of night shift work as “probably carcinogenic to humans”. This is based on limited evidence that night shift work causes breast, prostate, and colorectal cancer.
 
Night work is also linked with other health impacts including gastrointestinal disorders, cardiovascular disease, mental health issues and elevated risk of workplace incidents resulting in injuries.

WHSC can help

For our part, the WHSC offers Hours of Work training and a Shift Work hazard bulletin to help workplace parties better understand the health hazards posed by shift work, work at night and long hours. Equally important, WHSC training and resources focus on measures to better protect worker health, safety and work-life balance.
 
To learn more about WHSC training:
Call: 1-888-869-7950 and ask to speak to a training services representative
Visit: www.whsc.on.ca
Email: contactus@whsc.on.ca

Related WHSC resources
Exposure to night shift work when young raises the risk of breast cancer
New shift work resources highlights hazard of work/life imbalance