RELATIONSHIP BETWEEN PREVIOUS REPRODUCTIVE HISTORY AND CHEMICALLY INDUCED MAMMARY CANCER IN RATS

The relationship between a previous history of multiple pregnancies and/or lactations and 7,12‐dimethylbenzaanthracene‐induced mammary carcinogenesis was studied in groups of Sprague‐Dawley rats. All rats received the carcinogen at approximately 6 1/2 months of age. Virgin control rats which did not receive the carcinogen did not develop mammary cancer within 13 months although carcinogen‐fed virgin rats exhibited a mammary cancer incidence of 38.5%. A single pregnancy prior to feeding the carcinogen resulted in a decrease in the incidence and an increase in the mean time of appearance of the first palpable tumor. A single pregnancy followed by a subsequent nursing period reduced the percentage incidence and increased the mean time of appearance of the first palpable tumor when compared to the carcinogen‐fed virgin rats. A further increase in the number of pregnancies and/or lactations did not greatly influence the incidence of mammary cancer nor the number of cancers per rat. The mean time of tumor appearance was, however, increased. These data indicated that the relationship between party and carcinogen‐induced mammary cancer in the rat is similar to that existing in the human female but opposite to that found for the mouse. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company