Although difficult to study, factors in the aviation environment itself do not appear to affect reproductive outcomes, with the exception of radiation. Occupational factors such as long or irregular working hours, bending, standing, and lifting may have an adverse effect on reproductive outcomes, although a recent study only found that repeated bending for more than 1 hour a day after 34 week of pregnancy to be a significant risk. Early studies reported an increased risk of fetal death in flight attendants when compared to other employed women, but not to all women. Two recent studies suggested that menstrual irregularities and infertility, as well as fetal death, are more common in flight attendants. A meta-analytic study reported consistently higher rates of intrauterine fetal death and spontaneous abortions in flight attendants; other rates of adverse pregnancy outcomes were not elevated. Rates of induced abortions were lower, suggesting more effective use of birth control, which may affect the spontaneous fetal death rates. One case-control study of women with endometriosis has suggested that the observed increased risk of developing endometriosis was potentially correlated with circadian dysrhythmia, radiation, and possibly environmental chemical exposures.