Original article
Employment status as a confounder when assessing occupational exposures and spontaneous abortion

https://doi.org/10.1016/0895-4356(89)90162-5Get rights and content

Abstract

Studies of occupational exposure and spontaneous abortion may use pregnancies during which the mother was unemployed as part or all of the unexposed comparison group. Any type of maternal employment, however, may be a risk factor for spontaneous abortion, and potential confounder in occupational reproductive studies. This study evalutes the effect of employment in a cohort of pregnancies of 1535 women. Employed pregnancies had a significantly higher rate of spontaneous abortion (14.5%) than unemployed pregnancies (11.7%) (RR = 1.23, 95% CI = 1.02, 1.49). Gravidity acted as an effect modifier, as the employment effect was seen only in multigravidous pregnancies (RR = 1.38, 95% CI = 1.11, 1.72) and not primigravidous pregnancies (RR = 0.96). The effect persisted when an independent sample of one randomly selected pregnancy per woman was used for the analysis (RR = 1.27, 95% CI = 0.90, 1.79). The data were examined for confounding by other factors which could explain the excess in spontaneous abortion among employed pregnancies. The employment effect persisted with adjustment for other risk factors including maternal age, education, income, maternal diabetes, race, alcohol usage and smoking, and prior pregnancy ending in induced abortion. Stratifying by prior pregnancy loss eliminated the employment effect among those with prior loss (RR = 1.03) but enhanced the effect among those multigravidous without the risk factor (RR = 1.50, 95% CI = 1.15, 1.97). Selection bias, also, was explored as a possible explanation of this employment effect, but could not be substantiated. Assessment of a true exposure effect requires consideration of a potential employment effect either in the design or analysis.

References (25)

  • H.A. Risch et al.

    Risk factors for spontaneous abortion and its recurrence

    Am J Epidemiol

    (1988)
  • M. Joffe

    Biases in research on reproduction and women's work

    Int J Epidemiol

    (1985)
  • M.C. Marbury et al.

    Work and pregnancy

    J Occup Med

    (1984)
  • J.F. Murphey et al.

    Employment in pregnancy: prevalence, maternal characteristics, perinatal outcome

    Lancet

    (1984)
  • K. Hemminki et al.

    Spontaneous abortions and reproductive selection mechanisms in the rubber and leather industry in Finland

    Br J Ind Med

    (1983)
  • G. Axelsson

    Selection bias in studies of spontaneous abortion among occupational groups

    J Occup Med

    (1984)
  • S.G. Selevan

    Design considerations in pregnancy outcome studies of occupational groups

    Scand J Work Environ Health

    (1981)
  • G.K. Lemasters et al.

    Occupational styrene exposure for twelve product categories in the reinforced plastics industry

    Am Ind Hyg Ass J

    (1985)
  • G.K. Lemasters et al.

    Reproductive outcomes in women exposed to solvents in 36 reinforced plastics companies: 1. Menstrual dysfunction

    J Occup Med

    (1985)
  • G.K. Lemasters et al.

    Reproductive outcomes of women employed in the reinforced plastics industries: II. Lowered birthweight

    J Occup Med

    (1989)
  • G.K. Lemasters

    The reliability of reproductive and work history questionnaires

    Am J Epidemiol

    (1984)
  • S.G. Selevan

    Evaluation of data sources for occupational pregnancy outcome studies

  • Cited by (31)

    • Infertility

      2013, Women and Health
    • Infertility

      2012, Women and Health, Second Edition
    View all citing articles on Scopus
    View full text