The Menstrual, Reproductive, and Lactational Factors and the Risk of Breast Cancer in Korea: A Population Based Case-Control Study

Shin, Myung Hee; Yang, Jung Hyun; Choi, Kuk Jin; Ahn, Yoon Ok

Abstract

This study was conducted to assess the risk of breast cancer by menstrual, reproductive, and lactational factors in Korean women. A total of 210 women living in Seoul aged 30 to 60 with a first diagnosis of breast cancer from 2 teaching hospitals in Seoul, and 249 control subjects, randomly selected from the female pensioners of Korean Medical Insurance Company living in Seoul were interviewed. There was a increasing risk with the decrease of age at menarche, and the adjusted odds ratio(OR) of early menarche(age 14 years or less compared with over 17 years) was 1.61(95% Confidence Interval [CI]: 0.95-2.74). The longer the interval between menarche and regular menstruation, the less risk of breast cancer(OR=0.31, 95% CI: 0.16-0.60). The effect of these menarche factors were more prominent among premenopausal women. The adjusted OR for the second category(25-29 years old) of the age at first pregnancy and the age at the first full term pregnancy, compared to the reference category( 24 years old), were 0.52(95% CI: 0.30-0.92) and 0.58(95% CI: 0.32-1.06), respectively. The late age at the last full term pregnancy had significant protective effect (OR=0.29, 95% CI: 0.10-0.88), and the age itself was important rather than the interval between the first and last full term pregnancy. Factors related to breast feeding(BF) had protective effect consistently. Among parous women, in referent to no BF, women with 1 year of BF had adjusted OR of 0.34(95% CI: 0.17-0.68), while those with 1-3 years of BF had 0.35(95% CI: 0.18-0.67), 3-5 years had 0.29(95% CI: 0.12-0.74), and more than 5 years had 0.06(95% CI: 0.02-0.17). Age at menarche with regular cycle have stronger effect on especially pre-menopausal breast cancer among Korean women. Recent increase in breast cancer incidence in Korea would be explained by delayed age at first pregnancy or delivery, earlier age at menarche and last delivery, and decrease in brest feeding.