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In the present study, we explore whether a family history of breast cancer modifies time\u2010related effects of pregnancies, with special emphasis on the transient increase in risk of breast cancer shortly after birth. Our study cohort comprises 1,067,289 Norwegian women aged 20\u201374 years. The mean follow\u2010up time was 18 years. Incidence rate ratios were estimated by Poisson regression analyses of person\u2010years at risk. Of the 7,377 women diagnosed with breast cancer during follow\u2010up, a total of 828 (11%) had a mother or a sister with breast cancer diagnosis. Women with a family history of breast cancer had a 2\u20133\u2010fold higher risk of breast cancer than did women without any affected family member, highest for those with a relative diagnosed before they were 50 years. Similar to women without a familial excess risk, increasing parity was associated with an overall protective effect among women with a familial predisposition, regardless of age at diagnosis of the relative. Whereas women with no familial excess risk experienced a transient increase in risk mainly after late age births, women with a family history of breast cancer experienced an adverse effect of pregnancies also at younger ages. The present results give further support to the hypothesis that the adverse effect of a term birth can be explained by a promoting effect of pregnancy hormones. \u00a9 2006 Wiley\u2010Liss, Inc.<\/jats:p>","DOI":"10.1002\/ijc.22003","type":"journal-article","created":{"date-parts":[[2006,4,26]],"date-time":"2006-04-26T21:25:56Z","timestamp":1146086756000},"page":"1468-1474","source":"Crossref","is-referenced-by-count":22,"title":["Family history of breast cancer and short\u2010term effects of childbirths on breast cancer risk"],"prefix":"10.1002","volume":"119","author":[{"given":"Grethe","family":"Albrektsen","sequence":"first","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ivar","family":"Heuch","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Steinar","family":"Thoresen","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Gunnar","family":"Kv\u00e5le","sequence":"additional","affiliation":[],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"311","published-online":{"date-parts":[[2006,4,26]]},"reference":[{"key":"e_1_2_6_2_2","doi-asserted-by":"publisher","DOI":"10.1002\/(SICI)1097-0215(19970529)71:5<800::AID-IJC18>3.0.CO;2-B"},{"key":"e_1_2_6_3_2","doi-asserted-by":"crossref","first-page":"338","DOI":"10.1001\/jama.1993.03510030062035","article-title":"Family history, age, and risk of breast cancer. 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