Breast Cancer - Arab Medical Association Against Cancer

Epidemiology and risk assessment 4The older the age of menarche is, the lower the risk of breast cancer. For eachone-year delay in menarche, the risk decreases by around 5%. Other menstrualfactors such as cycle length and regularity have not been consistently related tobreast cancer risk.2. Parity and Age at First BirthCompared with nulliparous women, women with at least one full-term pregnancyhave, on average, around a 25% reduction in breast cancer risk. Increasingprotection is also seen with increasing numbers of full-term pregnancies, so thatwomen with five or more children have about half the risk of nulliparous women.The younger the age at first birth the greater the protection against breast cancer,this effect is independent of the total number of pregnancies. The exposure of themammary gland to sex hormones before a full-term pregnancy is critical becausethe breast tissue is especially susceptible to carcinogens due to the immaturity ofthe cells. A first child after 38 years old increases the risk by 2 fold as comparedto nulliparous women. The tendency, in western countries, to delay the 1 stpregnancy explains a part of the growing incidence of the last 30 years.Evidence about incomplete pregnancies, is less clear but suggests no large effecton breast cancer risk.3. Breast FeedingThe relation between breast-feeding and breast-cancer risk has been examined inmany studies; some have reported no association and others a reduced risk,particularly among pre-menopausal women. Recent studies in less developedcountries where the total duration of breast feeding can be very long, havereported clear protective effects. Protection has also been seen in some, but notall, studies in more developed countries. For example, the US Cancer and SteroidHormone Study found that breast feeding for a total of 25 months or morereduced the risk of cancer by 33% in over 4500 women studied.4. MenopauseThe age-specific incidence curve of female breast cancer suggests that themenopause has a protective effect. The relative risk of developing breast cancerfor a woman with natural menopause before age 45 is 0.73 compared with awoman with natural menopause between the ages of 45 and 54. The risk increasesby about 3% for each year older at menopause. The magnitude of this effect issimilar whether menopause occurs naturally or as a result of bilateral oophorectomy.