Besides fertility, overnight and shift work has also been linked to other women's health issues.
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. The data was analyzed in order to see if there is a connection between these factors and infertility. However, researchers aren't sure why or how jobs that require heavy lifting can negatively impact a woman's egg reserves.
"You need a study in the thousands", he told CNN, also citing the need to consider differences including socioeconomic status and testosterone levels in the women.
Researchers in the U.S. say previous investigations have shown that some work routines can influence how long it takes to get pregnant and a woman's ability to deliver a healthy baby at full-term.
However, the mechanism by which moving or lifting heavy loads could affect egg quality is still unknown.
The differences were even greater among those specifically working evening and night shifts, possibly because of disruption to the body clock, suggested the researchers, led by Dr Lidia Mínguez-Alarcón.
Research found that a physically demanding job or work schedules outside of normal office hours may lower a woman's ability to conceive.
The "reserve" refers to a woman's number of remaining eggs and level of follicle stimulating hormone (FSH), which rises as a woman ages and represents dwindling fertility.
The team at the Harvard T.H. Chan School of Public Health found that women working non-day shifts and those who had more physically demanding jobs had fewer mature oocytes retrieved after controlled ovarian hyperstimulation.
That same year, a study in PLOS Medicine concluded that rotating night-shift work is associated with an increase risk of type two diabetes in women.
The study showed that women who lift heavy object have 9% fewer eggs and 14% fewer mature eggs than women who don't have physically demanding jobs.
"It is hard to hypothesise a mechanism by which a physically demanding job may have a negative effect on ovarian reserve, as the number of eggs (oocytes) is determined at birth and lost progressively throughout life, with smoking having been shown to be the main toxin that significantly diminishes ovarian reserve".
On average, the women in the study had about 12 antral follicles in their ovaries, and nine mature eggs after treatment with fertility medications. They were quizzed about the jobs they did, as well as the level of physical exertion required for their roles and the hours and patterns worked.