Some jobs may be more conducive to pregnancy, finds a new study published in the journal Occupational and Environmental Medicine.

Researchers from the Harvard T.H. Chan School of Public Health, Harvard Medical School, Brigham and Women’s Hospital, and the Centers for Disease Control and Prevention sourced data from women who had previously participated in the Nurse’s Health Study in order to study the link between long work hours and physical labor. For four years, women reported the details of their work schedule and labor — think of standing on your feet for almost eight hours and frequently lifting objects that weigh at least 25 pounds — as well as how long it took them to get pregnant.

One year into the study, 16 percent of the total 1,739 women had not yet gotten pregnant. At the 24-month mark, so two years after the study initially began, five percent of women were still trying to conceive. When researchers compared womens’ work schedules, women working between 21 and 40-hour work weeks took less time to get pregnant than women working more than 40 hours per week.

Additionally, women who spent more time lifting or moving heavy objects took 50 percent longer to conceive than women not lifting or moving any objects. Even when researchers excluded women who were on their period (a known fertility impairment), heavy lifting-women still took 33 percent longer to get pregnant. These outcomes were even worse among overweight and obese women.

"Our results show that heavy work, both in terms of physical strain and long hours, appears to have a detrimental impact on female nurses' ability to get pregnant," Audrey Gaskin, lead author and a researcher at Harvard T.H. Chan, told Reuters’ Lisa Rapaport in an e-mail.

Rapaport cited most healthy couples can conceive within three to six months, though this duration is influenced by couples' age, lifestyle habits, and certain medical conditions.

There are a couple of explanations for these findings: Firstly, Gaskin said, it could be certain working conditions make it easier to get pregnant, in the same vein it’s possible women struggling to get pregnant are deliberately picking up extra shifts. Secondly, it takes a lot of women (and men) working late hours throughout the week.

“If this effect is real, it is likely due to the fact that these women are having less frequent intercourse due to their work demands,” Courtney Lynch, a specialist in reproductive health at Ohio State University in Columbus, told Rapaport.

Lynch’s advice for other nurses struggling to conceive? Have sex twice a week, not just during weekends. Maintaining a healthy lifestyle, such as regular exercise, can also increase chances of pregnancy.

Source: Gaskin AJ, et al. Work schedule and physical factors in relation to fecundity in nurses. Occupational and Environmental Medicine. 2015.