A similar pattern turned up among women who said they ever did heavy lifting at work: On average, they had one less mature egg than women who never moved heavy objects on the job.
The effect was more pronounced among overweight, obese and older women, the findings showed.
It’s biologically plausible that nighttime work could affect a woman’s egg quality, according to Minguez-Alarcon. In theory, she said, disruptions in the body’s normal circadian rhythms could be at play.
Shift work has been linked to a number of risks, including miscarriage and preterm birth, the researchers noted in their report.
However, Minguez-Alarcon said, more research is needed to understand why nighttime work may be tied to egg quality. There’s no clear explanation for why heavy lifting might matter, she added.
The study authors also pointed to another caveat: It’s not clear whether these findings might apply to women who are trying to conceive naturally.
The study was published online Feb. 7 in the journal Occupational Environmental Medicine.
For women undergoing fertility treatment, Grifo said, it’s important to keep the findings in perspective.
“A statistical study can never predict what will happen for an individual patient,” he said.
Do the findings suggest that, to be safe, women should avoid heavy exertion during fertility treatment?
Grifo said there’s no standard exercise advice for infertility patients, because there’s no clear evidence that it helps or harms. Some doctors, he noted, tell their patients to stop exercising during treatment — and some women may not feel like it due to side effects from ovary-stimulating medications.
Grifo said he generally recommends moderate exercise — particularly if a woman feels good being active.
“That might be her way of relieving stress,” he said. “Do you want to take that stress-reliever away from her?”
For the sake of their well-being, Grifo said, women should eat a healthy diet, get enough rest and stay hydrated. But ultimately, he stressed, they cannot “control” their response to fertility treatment.
“We don’t want [women] blaming themselves if it doesn’t work out,” Grifo said.