Table 3.

Reproductive function and breast cancer risk; E3N cohort study, 1990–1997

Reproductive events	No. of cases (n = 1718)	Person-years of follow-up	Multivariate RRa (95% CI)
Age at first FTPb (among parous) (years)
<22	232	94,468	1.00c
22–24	461	171,850	1.05 (0.89–1.23)
25–27	382	129,234	1.14 (0.96–1.35)
28–30	201	62,741	1.23 (1.01–1.50)
≥31	171	45,500	1.44 (1.17–1.79)
p for trendd			<0.0001
Number of FTPsb
0	271	75,732	1.00c
1	295	92,361	0.76 (0.61–0.95)
2	705	245,718	0.72 (0.59–0.88)
3	326	121,610	0.66 (0.54–0.81)
≥4	121	44,104	0.65 (0.51–0.83)
p for trendd			<0.0001
Duration of breastfeedingb (among parous)
0	440	143,654	1.00c
<3 months	796	282,256	0.92 (0.82–1.03)
≥3 months	211	77,883	0.84 (0.71–0.99)
p for trendd			0.12
Number of abortionsb
Never pregnant	218	59,626	1.05 (0.76–1.45)
Never aborted	846	290,565	1.00c
1	400	138,419	0.98 (0.87–1.11)
2	154	57,202	0.91 (0.76–1.08)
≥3	100	33,713	0.99 (0.80–1.22)
p for trendd, e			0.76

^aCox model with age (continuous) as time scale, including all variables, also adjusted for history of benign breast disease (Y/N), family history of breast cancer (Y/N), current body mass index (continuous variable), if ever married (Y/N), educational level (≤8, 9–12, 13–14, 15–16, ≥17 years of education), if OC ever used (Y/N), if ever treated for infertility (Y/N), or to regulate their cycles after menarche (Y/N).

^bMissing values (n_a breast cancer cases, n_b person-years of follow-up) were given the modal value; number of FTP: n_a = 20, n_b = 8,379; duration of breastfeeding: n_a = 78, n_b = 32,771; number of abortions : n_a = 2, n_b = 1,542; age at first FTP: n_a = 2, n_b = 1,555.

^cReference.

^dContinuous variable.

^eAmong pregnant women.