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The findings support the notion that "for those women seeking pregnancy, a healthy, regular routine is paramount," said Dr. Jill Rabin, chief of ambulatory care, obstetrics and gynecology at Long Island Jewish Medical Center in New Hyde Park, N.Y. She was not involved in the research.
In the new study, British researchers analyzed all studies on shift work and reproduction published between 1969 and 2013. The data from more than 119,000 women revealed that those working shifts (alternating shifts, evenings and nights) had a 33 percent higher risk of menstrual problems and an 80 percent higher risk of fertility problems than those who worked regular hours.
Women who worked only nights did not have a statistically increased risk of menstrual or fertility problems, but they did have an increased risk of miscarriage, the team found. This greater risk of miscarriage was not seen in women who worked nights as part of a shift pattern.
The study was presented July 9 at the annual meeting of the European Society of Human Reproduction and Embryology, in London.
According to the researchers, one possible explanation for the findings is that shift work's disruption of the body's circadian rhythm can affect the biological function of "clock genes," which have been shown to be associated with changes in biological functions.
The findings add to previous studies that found shift work can have harmful effects in later pregnancy, the authors noted in a society news release. If replicated in further research, these new "findings have implications for women attempting to become pregnant, as well as for their employers," wrote the team led by Dr. Linden Stocker, of the University of Southampton.
For her part, Rabin agreed that changes to the body's "clock genes" might throw off biological functions, including those linked to reproduction. And she said that while the study cannot prove that shift work causes these disruptions, the findings "are in keeping with other studies which found adverse effects of night shift work in late pregnancy."
Another expert said the stress of fluctuating work hours might also play a role.
"Stress, in and of itself, can alter the cycle of normal female hormone production -- ultimately contributing to poor reproductive outcomes, menstrual irregularities, low birth weight infants, infertility and preterm labor," said Dr. Kecia Gaither, director of maternal fetal medicine at Brookdale University Hospital and Medical Center in New York City. "Stress may manifest in various forms, such as a lack of sleep," she added.
Not everyone can change their jobs, however, so "women in these work circumstances should consider paying closer attention to some warning signs, such as menstrual irregularities (deviation from normal cycle frequency/duration), in order to prevent any future reproductive issues," Gaither said.
Other steps women might take to reduce their risk would be: seeing their doctor about any reproductive issues, cutting back on coffee or alcohol, "and perhaps simply finding time for consistent downtime with naps," she suggested.
-- Robert Preidt
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