Table 5.

Cumulative lifetime number of cycles and breast cancer risk. E3N cohort study, 1990–1997

Number of cycles	All womena			Excluding those who had ever used OCb
	No. of cases	Person-years of follow-up	Multivariatec RR (95% CI)	No. of cases	Person-years of follow-up	Multivariatec RR (95% CI)
<403	160	40,704	1.00d	108	29,298	1.00d
403–441	131	45,405	0.95 (0.75–1.20)	87	31,252	0.85 (0.64–1.14)
442–480	176	55,101	1.19 (0.95–1.49)	119	40,044	1.01 (0.77–1.32)
481–520	128	41,004	1.20 (0.94–1.54)	100	30,872	1.13 (0.85–1.51)
≥521	136	33,501	1.56 (1.22–1.99)	85	19,579	1.55 (1.15–2.10)
Impossible to compute	149	49,751	1.03 (0.81–1.30)e	97	35,399	0.89 (0.67–1.18)f
p for trend (excluding impossible to compute)			0.001			0.01

^aExcluding (number of cases/person-years): menopausal status unknown (106/53,273), age at menopause missing (161/76,294), age at menopause unknown because of HRT (571/184,492).

^bAlso excluding (number of cases/person-years): menopausal status unknown (70/28,732), age at menopause missing (94/45,673), age at menopause unknown because of HRT (303/91,765).

^cCox model with age (continuous) as time scale, adjusted for alcohol intake (0, <p50, ≥p50), history of benign breast disease (Y/N), history of infertility (Y/N), family history of breast cancer (Y/N), body mass index (<18.5, 18.5–25, >25), if OC ever used (Y/N), if ever married (Y/N), age at first FTP (nulliparae, <22, 22–24, 25–27, ≥28 years) and educational level (≤8, 9–12, 13–14, 15–16, ≥17 years of education).

^dReference.

^eReason (number of cases/person-years): cycles always irregular (100/32,569), missing data (49/17,182).

^fReason (number of cases/person-years): cycles always irregular (62/21,934), missing data (35/13,465).