American Journal of Epidemiology Vol. 145, No. 9: 817-825
Copyright © 1997 by The Johns Hopkins University School of Hygiene and Public Health
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Environmental Factors, Reproductive History, and Selective Fertility in Farmers' Sibships
1National Institute of Occupational Health, Oslo, Norway
2Medical Birth Registry of Norway, University of Bergen, Norway
3Section for Preventive Medicine, Institute of General Practice and Community Medicine, University of Oslo, Norway
Reprint requests to Dr. Petter Kristensen, National Institute of Occupational Health, P.O.B. 8149 Dep, N-0033 Oslo, Norway.
In a national study of births to farmers in Norway, grain farming was associated with short gestational age (21–24 weeks). An impact of selective fertility and maternal heterogeneity on the association was suspected but could not be assessed further in a traditional birth-based design. Thus, analyses based on the mother as the observational unit were performed. A total of 45, 969 farmers with a first birth in 1967–1981 were followed for subsequent births and perinatal mortality. A perinatal loss increased farmers' likelihood to continue to another pregnancy, but this selective fertility was less dominant than in the general population due to a higher baseline fertility. The effect of the mother's reproductive history on the grain farming-midpregnancy delivery association was analyzed in 59, 338 farmers with more than one single birth in 1967–1991. A history of preterm birth (<37 weeks) in previous or subsequent pregnancies both was an independent determinant of mid-pregnancy delivery and also increased the effect of grain exposure. Nongrain farmers with a history of only term births had 1.3 midpregnancy deliveries per 1, 000 births; grain farmers with a history of only term births had 1.8 cases per 1, 000 (odds ratio (OR) 1.4, 95% confidence interval (Cl) 1.0–1.9); nongrain farmers with a history of preterm birth had 6.8 cases per 1, 000 (OR 5.5, 95% Cl 4.0–7.6), whereas grain farmers with a history of preterm birth had 13.7 cases per 1, 000 (OR 11.0, 95% Cl 7.7–15.9). Selective fertility had only a marginal impact on the association. The study demonstrates that a maternally based design can contribute in the assessment of joint effects of environmental and maternal factors. Am J Epidemiol 1997; 145: 817-25.
agriculture; effect modifiers (epidemiology); environmental exposure; epidemiologic methods; fertility; gestational age; reproductive history