The role of pregnancy in maternal cancer risk: Epidemiologic evidence from the Nordic Countries Linked Birth and Cancer Registries Cohort Project

Abstract

The experience of pregnancy has a lasting impact, in many cases beneficial, on cancer risk in the mother. In the long term, breast, ovarian, and endometrial cancers are lower in parous women, and each pregnancy provides an additional risk reduction. Kidney cancer, in contrast, is elevated in parous women, while associations of parity with colorectal, thyroid, and pancreatic cancers are unclear. Timing of pregnancy matters, with a first birth at older ages compared with younger ages associated with an increased breast cancer risk, while endometrial and ovarian cancer risk is lower in women with an older versus younger age at last birth. Other characteristics of pregnancy are likely important but difficult to assess because of the time lag between pregnancy and cancer diagnosis, the potential for misclassification from recall in retrospective studies, and the rarity of many pregnancy conditions. Linking health-registries and pooling of data in the Nordic countries have provided an excellent opportunity to conduct epidemiologic research on rare pregnancy conditions and subsequent cancer, although legal restrictions have increasingly limited this approach. We hope that by identifying and describing associations with attributes of pregnancy, we may discover clues to the underlying biological mechanisms that lead to cancer development. Understanding the association of pregnancy and cancer risk will be of increasing importance as women have fewer pregnancies at later ages.