Summary report: early reproductive events and breast cancer workshop.

Introduction

The Early Reproductive Events and Breast Cancer Workshop convened February 24-26, 2003, and the outcomes of the meeting were reviewed and discussed at the joint meeting of the NCI Board of Scientific Advisors (BSA) and Board of Scientific Counselors (BSC) held March 3, 2003.

The Workshop was established to provide an integrated scientific assessment of the association between reproductive events and the risk of breast cancer. Participants represented a diversity of breast cancer expertise, including epidemiologists, clinicians, basic scientists and breast cancer advocates. The Workshop evaluated the current strength of evidence of the characteristics of pregnancy related to cancer (epidemiologic studies), the biologic changes resulting from pregnancy that may be involved in modifying breast cancer risk (clinical studies), and the biologic mechanisms identified (animal studies).

This report summarizes the epidemiologic, clinical and animal studies findings related to early reproductive events and breast cancer risk, and each finding is given a Strength of Evidence Rating.** Gaps in research knowledge for each scientific area are identified, and recommendations for future research directions are provided.

Epidemiologic Findings

* Early age at first term birth is related to lifetime decrease in breast cancer risk. (1)

* Increasing parity is associated with a long-term risk reduction, even when controlling for age at first birth. (1)

* The additional long-term protective effect of young age at subsequent term pregnancies is not as strong as for the first term pregnancy. (1)

* A nulliparous woman has approximately the same risk as a woman with a first term birth around age 30. (1)

* Breast cancer risk is transiently increased after a term pregnancy. (1)

* Induced abortion is not associated with an increase in breast cancer risk. (1)

* Recognized spontaneous abortion is not associated with an increase in breast cancer risk. (1)

* Long duration of lactation provides a small additional reduction in breast cancer risk after consideration of age at and number of term pregnancies. (1)

* Pregnancy-induced hypertension is associated with decreased breast cancer risk. (2)

* Maternal DES exposure is associated with an increase in breast cancer risk. (3)

Epidemiologic Gaps

* By what mechanism does pregnancy at an early age protect against breast cancer?

* Do pregnancy and age at pregnancy modify radiation-induced breast cancer risk?

* What are...

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