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In a study of women undergoing fertility treatment, researchers found that those who worked at night or did heavy lifting on the job tended to have fewer "mature" eggs. In theory, that could lower their chances of having a baby.
However, experts stressed that the findings have to be viewed with caution.
For one, they do not prove that night shifts or heavy lifting are to blame, said lead researcher Lidia Minguez-Alarcon. She is a research fellow at the Harvard School of Public Health in Boston.
It's possible, she explained, that some women with those jobs are exposed to other environmental factors that might affect their "egg quality."
Beyond that, the study did not show that women's job situations directly affected their chances of having a baby, said Dr. James Grifo, director of the NYU Langone Fertility Center in New York City.
Grifo, who was not involved in the study, was concerned the findings could cause women undue stress or guilt if their fertility treatment does not work out.
In fact, Grifo said, the study results can point only to an association between certain job situations and a measure of fertility.
"Do you tell women to quit their jobs or to stop lifting based on this?" Grifo said. "I don't think so."
During IVF, women take medications that stimulate the ovaries to produce as many mature eggs as possible. The eggs are then retrieved so they can be fertilized in a lab dish.
That allowed Minguez-Alarcon and her colleagues to get a precise count of the mature eggs each patient produced.
Overall, the investigators found, women who worked night shifts or rotating shifts tended to produce fewer mature eggs compared to women who worked only during the day -- about two fewer, on average.
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.
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.
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.
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