Pregnancy causes pituitary enlargement, apparently through an estrogen-related hyperplasia of the prolactin-secreting acidophilic cells in the anterior lobe (/-3). Since cell proliferation is an important background factor in neoplasia (4), pregnancy-associated pituitary hyperplasia might be expected to confer an increased risk of pituitary adenoma. To evaluate this hypothesis, we linked data on women born from 1925 through 1960 from the Swedish Cancer Registry to data from the Swedish Fertility Registry, as previously described (5).

All diagnosed pituitary tumors [International Classification of Diseases, 7th Revision, code No. 195.3 (6)] are reported to the Swedish Cancer Registry (7). A total of 350 case patients older than age 15 years at diagnosis from January 1, 1958, through December 31, 1984, were identified and linked to the Swedish Fertility Registry. Each case patient was matched for year and month of birth to five control subjects (n= 1750), who were not themselves reported to the Swedish Cancer Registry with a pituitary adenoma diagnosis. Odds ratios (ORs) were calculated by conditional logistic regression; tests for trend were conducted by including categorical variables as continuous variables.