Cancer facts: abortion, miscarriage, and breast cancer risk.

National Cancer Institute *

Introduction

A woman's hormone levels normally change throughout her life for a variety of reasons, and these hormonal changes can lead to changes in her breasts. Many such hormonal changes occur during pregnancy, changes that may influence a woman's chances of developing breast cancer later in life. As a result, over several decades a considerable amount of research has been and continues to be conducted to determine whether having an induced abortion, or a miscarriage (also known as spontaneous abortion), influences a woman's chances of developing breast cancer later in life.

Current Knowledge

In February 2003, the National Cancer Institute (NCI) convened a workshop of over 100 of the world's leading experts who study pregnancy and breast cancer risk. Workshop participants reviewed existing population based, clinical, and animal studies on the relationship between pregnancy and breast cancer risk, including studies of induced and spontaneous abortions. They concluded that having an abortion or miscarriage does not increase a woman's subsequent risk of developing breast cancer. A summary of their findings, titled Summary Report: Early Reproductive Events and Breast Cancer Workshop, can be found at http://www.cancer.gov/ cancer info/ere-workshop-report.

Related NCI Materials

* Pregnancy and Breast Cancer Risk fact sheet (currently in preparation)

* What You Need To Know About[TM] Breast Cancer http://www.cancer.gov/cancerinfo/wyntk/breast

Background

The relationship between induced and spontaneous abortion and breast cancer risk has been the subject of extensive research beginning in the late 1950s. Until the mid-1990s, the evidence was inconsistent. Findings from some studies suggested there was no increase in risk of breast cancer among women who had had an abortion, while findings from other studies suggested there was an increased risk. Most of these studies, however, were flawed in a number of ways that can lead to unreliable results. Only a small number of women were included in many of these studies, and for most, the data were collected only after breast cancer had been diagnosed, and women's histories of miscarriage and abortion were based on their "self-report" rather than on their medical records. Since then, better-designed studies have been conducted. These newer...