The influence of pregnancies on the incidence of cancer in mice

Abstract

In order to analyze still further the various factors causing the spontaneous development of cancer of the breast in mice we undertook investigations into the influence of pregnancy in various strains of mice.

Accordingly we kept a number of mice from various strains after they were weaned separated from males throughout their life. Other mice from the same strains were allowed to breed in the usual manner. A few months after we had begun our experiments we received a recent communication by Bashford, who stated that he did not notice any influence of pregnancies on the incidence of cancer in mice.

Our results are as follows:

I. English Strain.—Never bred mice.

Group A (24 non-breeding mice): 46 per cent. have tumors. The corresponding percentage in bred English mice varies in different groups between 62 per cent. and 75 per cent.

91 per cent. of the tumors in never-bred mice occurred in mice older than 12 months, 36 per cent. in mice above 17 months of age, while in breeding English mice the corresponding percentages were 32 per cent. and 4.6 per cent respectively.

Group B (58 non-breeding mice): Experiment not yet finished. 17 per cent. of the mice have had tumors so far, almost all of them between 10 and 12 months of age. All others, 22 of which are already more than 12 months old, have not yet had tumors In control mice 68 per cent. had tumors at or under 12 months of age. We find therefore that even in English mice that are prevented from breeding a considerable number of tumors occur, but that the number of mice affected by cancer is distinctly decreased and that the age at which the tumors develop is higher in non-breeders than in breeders.