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Shift work, nitrous oxide exposure and subfertility among Swedish midwives.

BACKGROUND
Shift work and nitrous oxide exposure have both been suspected of having adverse influence on the reproductive performance of health workers. Time to pregnancy has been suggested as a sensitive measure of fecundity in occupationally exposed groups. We investigated the effects of shift work and nitrous oxide exposure on the fertility of Swedish midwives.


METHODS
A questionnaire was mailed to all members of the Swedish Midwives Association who were born 1940 or thereafter, 3985 in all. Eighty-four per cent responded. Detailed information on the number of menstrual cycles required to achieve pregnancy and the working conditions during that period were obtained concerning the most recent, planned pregnancy occurring after 1983. The per cycle probability of becoming pregnant was calculated for each exposure category, and the relation to the unexposed was expressed as fecundability ratios.


RESULTS
Midwives who worked two-shift, three-shift rotas, or only nights had reduced fertility compared to those working in the day time. The fecundability ratios were 0.78 (95% confidence interval [CI]: 0.65-0.94), 0.77 (95% CI: 0.60-0.98), and 0.82 (95% CI: 0.64-1.03), respectively, after adjustment for covariates. No effect of nitrous oxide exposure was noted except in the small group reporting that they assisted at more than 30 deliveries per month when nitrous oxide was used (fecundability ratio = 0.64; 95% CI: 0.44-0.95).


CONCLUSION
Shift work and frequent, high occupational exposure to nitrous oxide may have a negative influence on the ability of women to become pregnant.