Multiple pregnancies among women engaged in agriculture in Norway

Sammendrag

Background.

Methods.

Results.

!30 years, and were more likely to undergo in

vitro fertilization (IVF). Combined grain farming and seaso nal late blight warnings did not influence the

prevalence 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 pregnancies

than mothers in non-farming sectors of the agricultural population. After the late 1970s this situation

was reversed, mainly because the latter group consisted of more older first-time mothers, and probably

with a higher need of assisted fertility treatment. Interpretations are difficult concerning the inverse relation

between 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. The

prevalence of DZ pregnancies was declining until the late 1970s, and increased thereafter. The agricultural

population 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 the

farm holding. Farmers, in particular active farmers, had more DZ pregnancies than non-farmers early in the

study period; this ratio was reversed late in the study period. Non-farmers had a much higher temporal

increase in the proportion of first-time mothers who were

By record linkage of national registers in Norway, we identified 246,043 farm holders and

spouses 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 of

multiple pregnancy, subdivided in DZ and monozygotic (MZ) pregnancies, were examined in strata of

several determinants. Exposure, defined as the combination of grain farming and categories of seasonal

fungal 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.

SUMMARYThe worldwide occurrence of dizygotic (DZ) multiple pregnancies has increased since 1980

as a consequence of assisted fertilization techniques. An opposite influence by environmental factors yet

unidentified has been a suspected explanation of decreased multiple pregnancies in several countries

during 1960-1980. The aim of our study was to describe multiple, in particular DZ, pregnancy patterns in

the Norwegian agricultural population between 1967 and 1991, and to investigate the hypothesis that grain

production under climatic conditions favoring fungal growth causes a decline in DZ pregnancies.