Shift work, heavy lifting linked to less successful in vitro fertilization

Fertility treatments are arduous, with many factors contributing to the chance of conceiving, but now U.S. researchers say shift work or heavy lifting on the job may reduce the odds for success.

Shift work disrupts sleep and the body's internal clock and has been associated with high blood pressure, cardiovascular disease and some cancer.

• Night shift effects on brain comparable to chronic jet lag
• Night shift workers face increased breast cancer risk

Researchers monitored indicators of capacity to reproduce among 473 women with an average age of 35 who attended a fertility clinic in Boston as part of the Environment and Reproductive Health or EARTH study.

Previous studies have suggested a link between work schedules, heavy lifting at work and capacity to reproduce. But they weren't able to take direct measures, such as levels of reproductive hormones or immature eggs, called oocytes, remaining in the ovary.

In the study, published in the journal Occupational and Environmental Medicine, there were 190 women who moved or lifted heavy objects at work.

"We found that women who are working non-day shifts as well as those who are moving or lifting heavy objects at work had significantly fewer total eggs retrieved as well as mature eggs retrieved when undergoing in-vitro fertilization," study author Audrey Gaskins, a research associate at Harvard T.H. Chan School of Public Health and an instructor at Harvard Medical School, said in an interview.

"And this is important because only the mature eggs are capable of developing into healthy embryos needed to sustain a pregnancy. So in theory if a woman has fewer mature eggs that would lower her chances of becoming pregnant."

The lower egg count among women who work outside of the regular hours may be related to a disrupted biological clock, the researchers speculated.

Cause and effect not proven

The study was observational and no cause-and-effect relationships can be drawn.

Previous studies in European and Asian women also pointed to associations between shift work and reduced ability to conceive, while other research hasn't. 

For instance, it could be that the amount and type of work a women does could be related to other aspects of her life, such as socioeconomic status, that made her less fertile.

Dr. Tom Hannam, a fertility specialist in Toronto, gives the example of a patient who cleans hotel rooms, a physically demanding job. 

"She has very little control over the hours that she works," Hannam said. "She's forever being called in at times which make it difficult, so she's really struggling with fertility. It's not the individual or how much she wants to have a baby, it's the job that she has. It's not the heavy lifting. It's the lack of control she has in her life which is making it difficult for her."

Link to fertility unclear

Gaskins acknowledged that many of the women coming to the clinic in the study were of slightly higher socioeconomic status than average. Women in the study who reported moving or lifting heavy objects at work were less educated on average than those who reported never doing so.  

It's also difficult to hypothesize a mechanism by which a physically demanding job could affect the ovarian reserve, as the number of eggs or oocytes is determined at birth, Prof. Adam Balen, chair of the British Fertility Society, said in statement.

If a woman is concerned, Gaskins suggested she talk to her supervisor. "Explore the possibility of discontinuing night shifts … or lowering the frequency of lifting heavy objects."

Quitting a job in order to conceive would be a dramatic step with possible unintended consequences, Hannam said.

"Wouldn't you hope to work in an environment that there is support? So that if you have a medical condition … your workplace is able to find a solution to give you the time and the space to take the treatment that you need? I don''t think this is just true for fertility. It's true for all health concerns."

Gaskins hopes to continue her research by exploring whether women who stop working shifts or heavy lifting at work show better ovarian measures in the short term. 

The research was funded by the U.S. National Institutes of Health.