Abstract
Background. Methods. Results. !30 years, and were more likely to undergo invitro fertilization (IVF). Combined grain farming and seaso nal late blight warnings did not influence theprevalence of DZ pregnancies. Unexpectedly, the same climate variable, in combination with horticulture,was negatively associated with DZ pregnancies (PR 0.3; 95% confidence interval 0.1–0.7).Conclusion. Active Norwegian farmers seem to have a somewhat higher natural occurrence of DZ pregnanciesthan mothers in non-farming sectors of the agricultural population. After the late 1970s this situationwas reversed, mainly because the latter group consisted of more older first-time mothers, and probablywith a higher need of assisted fertility treatment. Interpretations are difficult concerning the inverse relationbetween DZ pregnancy and climatic conditions in horticulture.We identified 2,131 (11.2 per 1,000) multiple pregnancies, 1,322 (6.9) were classified as DZ. Theprevalence of DZ pregnancies was declining until the late 1970s, and increased thereafter. The agriculturalpopulation of Norway is heterogeneous along two dimensions, with a difference between farmers and nonfarmers(mainly engaged in forestry), and a difference according to degree of maternal work input on thefarm holding. Farmers, in particular active farmers, had more DZ pregnancies than non-farmers early in thestudy period; this ratio was reversed late in the study period. Non-farmers had a much higher temporalincrease in the proportion of first-time mothers who were By record linkage of national registers in Norway, we identified 246,043 farm holders andspouses born between 1925 and 1971 in agricultural censuses and the population register. In this population,190,258 pregnancies were recorded in the Medical Birth Registry 1967-91. The prevalences ofmultiple pregnancy, subdivided in DZ and monozygotic (MZ) pregnancies, were examined in strata ofseveral determinants. Exposure, defined as the combination of grain farming and categories of seasonalfungal warnings, was based on data on farm activity and on local fungal warnings in the growth seasons.Adjusted prevalence ratios (PR) served as estimates of association.SUMMARY The worldwide occurrence of dizygotic (DZ) multiple pregnancies has increased since 1980as a consequence of assisted fertilization techniques. An opposite influence by environmental factors yetunidentified has been a suspected explanation of decreased multiple pregnancies in several countriesduring 1960-1980. The aim of our study was to describe multiple, in particular DZ, pregnancy patterns inthe Norwegian agricultural population between 1967 and 1991, and to investigate the hypothesis that grainproduction under climatic conditions favoring fungal growth causes a decline in DZ pregnancies.
Highlights
Throughout the world, prevalences of multiple pregnancies have varied considerably during the last decades.[1]
The purpose of the present study was twofold: to describe secular patterns of multiple, in particular DZ, pregnancies in the total Norwegian agricultural population and subgroups of this population, secondly, to pursue the hypothesis that conditions favoring fungal growth and mycotoxin formation in grain farming are inversely related with DZ pregnancies
Pregnancies after in vitro fertilization (IVF) treatment are registered since the late 1980s by the Medical Birth Registry in cooperation with the IVF laboratories.[7]
Summary
Throughout the world, prevalences of multiple pregnancies have varied considerably during the last decades.[1] Hormonal induction of ovulation and IVF have increased overall multiple birth rates since the 1980s.2-7. In the decades preceding the use of these fertility treatment techniques, Western European and several other countries experienced a decline in dizygotic (DZ) twin rates.[8, 9] This was a matter of concern, because the decline was assumed to be a sign of declining fecundity. The causes of the observed DZ decline remained obscure; one hypothesis being that environmental factors disturb the hormonal regulation of ovulation.[9, 10] In Norway, the prevalence of multiple pregnancies was stable (about 10 per 1,000) between 1967 and the mid-1980s, but increased by 30% in the years to follow.[11]
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