Jobs that involve carrying heavy loads — with effects seen stronger among overweight, obese, and older women — and working night or rotating shifts could reduce a woman’s fertility, a new study has warned.

A team from Harvard T.H. Chan School of Public Health made a link between reproductive capacity and these occupational factors, but the underlying cause remains unknown. Yet women who are in a reproductive age and are actively trying to conceive should consider these findings, according to the researchers.

Lowered Egg Count

The team studied almost 500 women who sought infertility treatment at Massachusetts General Hospital from 2004 to 2015, and compared their jobs’ physical demand and schedules against four specific biomarkers, or genes in the body, associated with fecundity or the ability to reproduce.

These biomarkers are the number of immature eggs in the body and mature eggs that can develop into healthy embryos, as well as levels of follicle-stimulating hormone (FSH) and estrogen.

The results: the heavier the lifting or moving of heavy objects at work, the lower the amount of antral follicles and eggs. Those who reported lifting heavy objects had 8.8 percent fewer eggs and 14.1 percent fewer mature eggs versus those who never lifted or moved heavy stuff around.

This was discovered more pronounced among subjects who were overweight, obese, and ages 37 and above.

Working at night or on rotating shifts, too, also appeared inversely related to the number of eggs. The team speculated it may be related to the disruption of one’s body clock or circadian rhythm.

"Our study is the first to show that occupational heavy lifting and non-day shifts may be adversely affecting egg production and quality, rather than accelerating ovarian aging,” explained senior author Audrey Gaskins in a statement, also pointing to a disrupted stress-response system brought about by factors like obesity.

Other Expert Findings

Reproductive endocrinologist Channa Jayasena, who was not part of the study, said that the study was interesting but there should be a bigger sample size than the less than 500 used in the research.

“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.

Jayasena, however, agreed with the researchers that circadian rhythm may be at play in the differences found in shift work, with each body part — including the ovaries — having its own circadian rhythm. Previous research has reflected the potential ills of shift work, including a greater risk of obesity and heart disease.

The findings were discussed Feb. 7 in the journal Occupational and Environmental Medicine.

A separate study from the National Institutes of Health (NIH) pinpointed obesity as a factor influencing a couple’s ability to conceive. It showed that if both the partners are obese, then the couple may take almost 55 percent to 59 percent more time to conceive compared to non-obese couples.

In most cases, obesity is tied to less successful fertility treatments. Thus experts are urged to make couples conscious of body compositions and weight issues that affect pregnancy during counseling sessions.

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