separately for nulliparous women. The results are inconsistent. Two studies found increasing risk with longer duration of use in young women (Stadel et al., 1989; U.K. National Case-Control Study Group, 1989), and another (Meirik et al., 1986) found significantly elevated relative risks in nulliparous women regardless of duration of use. Other equally well-designed and well-conducted studies, however, found no increase in risk in nulliparous users.

Risk of breast cancer increases with the age of a woman at the time of her first live birth. Table B-10 shows relative risks in relation to use of oral contraceptives in women of varying ages at the time of their first live birth. No trends of increasing or decreasing relative risk associated with age at first birth are seen in any study.

Women of high parity tend to be at low risk of breast cancer. Table B-11 shows that no studies have shown a trend in relative risks in users of oral contraceptives related to number of children that a woman has had.

Table B-12 shows relative risks in users of oral contraceptives who do and do not have a family history of breast cancer. None of the studies summarized showed appreciable differences in the magnitude of relative risks between women with and without various affected relatives.

Some studies have shown that obese women are at increased risk of breast cancer, especially in their postmenopausal years. As shown

TABLE B-8 Relative Risk of Breast Cancer in Women of Various Socioeconomic Strata Who Have Ever Used Oral Contraceptives

First Author (Date)

Measure of Socioeconomic Status

Level

Relative Risk

Confidence Interval (95%)

Miller (1989)

Years of education

<13

2.3

Includes 1.0 a

   

13-16

1.6

Includes 1.0

   

>17

2.8

Includes 1.0 b

WHO c (1990)

Socioeconomic index d

4 (low)

1.86

1.07,3.24

   

3

1.13

0.91,1.40

   

2

1.14

0.94,1.39

   

1 (high)

1.12

0.94,1.32

a In multivariate analysis, relative risk = 2.7 (95 percent confidence interval [CI] excludes 1.0).

b In multivariate analysis, relative risk = 3.9 (95 percent CI excludes 1.0).

c WHO = World Health Organization.

d Based on years of education and occupation.



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