Parity and susceptibility to cancer.
Since fetal antigens are produced by tumour cells and pregnant women may have had contact with certain of these antigens, it is important to establish whether parous women have an altered susceptibility to cancer. Cancers of the breast, ovary and endometrium are less common in parous than nulliparous women. The more children a woman has, the greater is her protection from these cancers. The protection lasts throughout life. The only time when the reverse seems to be true is at childbearing ages, suggesting that in the short term pregnancy may increase cancer risk. There is no direct evidence that pregnancy protects against cancers of non-reproductive sites. Nevertheless, for most sites women have lower incidence rates than do men, and married women have lower rates than single women. The incidence of cancers of the stomach, gall bladder and thyroid, and possibly colon cancer and melanoma, may be increased in parous women (although the evidence is contradictory). Except for stomach cancer, tumours at these sites are more common in women than men. Most of the explanations for the relationship between childbearing and cancer have stressed the role of endocrine and metabolic factors. Immunological factors may well be relevant and need to be taken into consideration in future studies of cancer in human populations.