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 sele ctive fertility and maternal heterogeneity on the association was susp ected but could not be assessed further in a traditional birth-based d esign, Thus, analyses based on the mother as the observational unit we re performed, A total of 45,969 farmers with a first birth in 1967-198 1 were followed for subsequent births and perinatal mortality, A perin atal loss increased farmers' likelihood to continue to another pregnan cy, but this selective fertility was less dominant than in the general population due to a higher baseline fertility. The effect of the moth er's reproductive history on the grain farming-midpregnancy delivery a ssociation was analyzed in 59,338 farmers with more than one single bi rth in 1967-1991. A history of preterm birth (<37 weeks) in previous o r subsequent pregnancies both was an independent determinant of midpre gnancy delivery and also increased the effect of grain exposure, Nongr ain farmers with a history of only term births had 1.3 midpregnancy de liveries per 1,000 births; grain farmers with a history of only term b irths had 1.8 cases per 1,000 (odds ratio (OR) 1.4, 95% confidence int erval (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 farmer s with a history of preterm birth had 13.7 cases per 1,000 (OR 11.0, 9 5% CI 7.7-15.9). Selective fertility had only a marginal impact on the association, The study demonstrates that a maternally based design ca n contribute in the assessment of joint effects of environmental and m aternal factors.