An international collaborative study has been carried out to test the hypothesis that prolonged lactation protects women against cancer of the breast. While pregnancy itself seemed to confer some protection against breast cancer in all areas studied, no consistent differences in duration of lactation were found between breast cancer patients and unaffected women, once the fact that breast cancer patients have fewer pregnancies had been allowed for. Even in areas where some women had lactated for a total of 5 years or more, such women occurred proportionately no less frequently among breast cancer patients than among unaffected women. In the light of this and other recent evidence, it is unlikely that lactation has any protective effect against breast cancer in women, and other explanations must be sought for the remarkable international differences in the frequency of this disease.

Adult↗

PubMed1970

Lactation and reproductive histories of breast cancer patients in Tokyo, Japan.

Epidemiological characteristics of breast cancer have been studied in Tokyo, an area of relatively low breast cancer rates, as part of an international collaborative study. Breast feeding of infants is customary for Japanese women, but even very prolonged lactation did not appear to be associated with a reduction in breast cancer risk. Late age at menarche and early age at first pregnancy were both associated with reduced breast cancer risk. The protective effect of early pregnancy seems much greater than has previously been recognized, and may indeed be greater in Japan than in other areas. However, the average age at first pregnancy is not lower in Tokyo than in areas than in areas where breast cancer rates are high.

Adult↗

PubMed1970

Age at first birth and breast cancer risk.

An international collaborative study of breast cancer and reproductive experience has been carried out in 7 areas of the world. In all areas studied, a striking relation between age at first birth and breast cancer risk was observed. It is estimated that women having their first child when aged under 18 years have only about one-third the breast cancer risk of those whose first birth is delayed until the age of 35 years or more. Births after the first, even if they occur at an early age, have no, or very little, protective effect. The reduced risk of breast cancer in women having their first child at an early age explains the previously observed inverse relationship between total parity and breast cancer risk, since women having their first birth early tend to become ultimately of high parity. The association with age at first birth requires different kinds of etiological hypotheses from those that have been invoked in the past to explain the association between breast cancer risk and reproductive experience.

Adolescent↗

PubMed1969