Women who worked more than 40 hours weekly took around 20% longer to get pregnant than those who worked 21-40 hours weekly, according to the study.
Lead study author Dr. Audrey J. Gaskins, of the Harvard T.H Chan School of Public Health in Boston, MA, and colleagues reached their findings by analyzing the data of 1,739 women of an average age of 33 who were part of the 2010-2014 Nurses' Health Study.
All women included in the analysis were attempting to get pregnant. At study baseline, participants completed a questionnaire that detailed their work schedules and physical labor.
The team assessed participants every 6 months to determine how long it was taking them to conceive.
Around 44% of the women included in the study were overweight or obese, according to the researchers, while 22% were current or former smokers.
Most of the women worked days or nights only, while 16% worked rotating shifts. More than 30% of women reported working for more than 8 hours daily, while 40% said they lifted heavy loads up to five times each day.
Working more than 40 hours weekly may increase time to conception by 20%
After 12 months, 16% of study participants had not become pregnant, while 5% had not conceived after 2 years.
Compared with women who worked 21-40 hours each week, those who worked more than 40 hours weekly took around 20% longer to get pregnant, according to the results.
In addition, the team found women who moved or lifted a minimum of 25 pounds in weight more than 15 times a day took around 50% longer to conceive than women who never lifted or moved heavy loads.
When accounting for irregular menstrual cycles - a factor known to impact fertility - the researchers found women who regularly lifted or moved heavy loads still took around 33% longer to conceive.
The association between heavy lifting and longer duration of pregnancy attempt was more prominent among women who were overweight or obese, the researchers report.
The team found no link between frequency of night shifts and the time it took to conceive, nor was there an association between duration of rotating or non-rotating night work and time to conception.
Commenting on the study findings, Dr. Gaskins told Reuters:
"Our results show that heavy work, both in terms of physical strain and long hours, appears to have a detrimental impact on female nurses' ability to get pregnant."
Speaking to Medical News Today, Dr. Gaskins said the researchers were unable to identify the exact drivers behind their findings.
"While we tried to control for many variables that could potentially explain this association - such as menstrual cycle regularity, sleep duration and other potentially harmful lifestyle and work exposures - none of these factors seemed to completely explain these relationships," she said.
"There were some variables, however, that we lacked in our study, such as frequency of sexual intercourse and work-related strain and fatigue," she continued. "Because of this, it is hard to determine whether these relationships are being driven by other correlated lifestyle exposures or something more biological, such as dysregulation of circadian rhythm."
Talking to Reuters, Courtney Lynch, a reproductive health specialist at Ohio State University, says the findings may simply be caused by a reduction in sexual intercourse due to physical exhaustion from lifting heavy loads.
In addition, because the study only included women who were nurses, Dr. Gaskins told MNT that it is difficult to say whether their findings apply to women more generally.
Ways to get pregnant faster
The time it takes to get pregnant varies, depending on factors such as age, lifestyle habits and medical conditions that may impact fertility. The majority of couples, however, become pregnant within 3 months.
Lynch says there are a number of ways to help speed up time to conception. She recommends couples trying to get pregnant should have sex at least twice weekly, and women should avoid stress, smoking and maintain a healthy weight.
According to BabyCenter, women should also time sex with ovulation in order to increase their pregnancy chances, citing one study that found women who did so significantly increased their time to conception.
Earlier this month, Medical News Today reported on a study detailing a new in vitro fertilization (IVF) approach that researchers say could lead to more effective fertility treatment for older women.