Table 3A.

Incidence rate ratio (IRR, referred to as relative risk) of breast cancer among women aged 15-34 years at study start, according to duration of current combined hormonal contraception (HC) use (1), time since most recent use (2), and cumulative duration (3).

	No of breast cancer events	No of person-years	Incidence rate	Age-adjustedincidence rate/100,000 person-years	IRR	95% CI	p	No of breast cancer events	No of person-years	IRR	95% CI	p
	Crude estimates			Adjusted estimates– Model 1a				Model 2b
Never used HC	1419	6,322,450	22.4	20.4	1.00	Reference	NA	851	2,459,880	1.00	Reference	NA
Duration of combined HC useb
0 < 5 years	114	1,345,829	8.5	25.9	1.39	1.14–1.69	<0.01	77	4,47,054	1.32	1.03–1.68	0.03
5 < 10 years	132	1,213,245	10.9	20.8	1.06	0.88–1.13	0.53	45	3,35,251	0.66	0.49–0.90	<0.01
≥ 10 years	68	3,18,304	21.4	16.8	0.78	0.61–1.00	0.05	19	41,230	0.70	0.44–1.10	0.05
Time since most recent use
0 < 5 years	261	7,84,184	33.3	39.6	1.66	1.45–1.90	<0.01	221	5,00,231	1.72	1.47–2.00	<0.01
5 < 10 years	293	1,089,677	26.9	39.5	1.32	1.16–1.50	<0.01	172	4,17,879	1.12	0.95–1.33	0.16
≥ 10 years	238	6,92,160	34.4	18.2	0.85	0.74–0.98	0.02	104	1,77,845	0.77	0.63–0.96	0.02
Cumulative duration of combined HC usec
0 < 5 years	68	1,237,573	5.5	21.3	1.30	1.00–1.67	0.04	35	3,47,758	1.03	0.73–1.46	0.87
5 < 10 years	171	1,290,061	13.3	23.9	1.23	1.04–1.45	0.01	80	4,05,607	0.99	0.78–1.25	0.95
≥ 10 years	75	3,49,744	21.4	15.8	0.75	0.59–0.94	0.01	26	70,170	0.67	0.45–0.99	0.04

^aAdjusted for age at start of each exposure, level of education, place of birth, age at first pregnancy registered for antenatal care, number of children, having received ovulation stimulating treatment, and any diagnosis of infertility, polycystic ovarian syndrome, or endometriosis.

^bAdjusted for the same covariates as Model 1, including for body-mass index (BMI) and smoking (available for parous women only; 37% of the study population).

^cCumulative time per person during the study period.