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% Cl 7.7-15.9). (...)