Environmental factors, reproductive history, and selective fertility in farmers' sibships.

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 midpregnancy 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 (CI) 1.0-1.9); nongrain farmers with a history of preterm birth had 6.8 cases per 1,000 (OR 5.5, 95% CI 4.0-7.6), whereas grain farmers with a history of preterm birth had 13.7 cases per 1,000 (OR 11.0, 95% CI 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.