Table 1. Number of breast cancer diagnoses (no. of cases) and person-years (P-yr) by reproductive factors during follow-up of 1.7 million Norwegian women aged 20–74 years.

	Total		1st birth		2nd birth		3rd birth		4th birth		5th birth
	No. of cases	P-yr (× 104)	No. of cases	P-yr (× 104)	No. of cases	P-yr (× 104)	No. of cases	P-yr (× 104)	No. of cases	P-yr (× 104)	No. of cases	P-yr (× 104)
Parity a
0	3306	1066.1
⩾1	19 584	2722.3	3543	698.0	8682	1125.6	5160	619.4	1757	213.6	442	65.7
Time since most recent birth (years) b
<5	1403	981.9	232	357.0	582	373.3	402	178.9	152	56.1	35	16.5
5–9	2144	503.0	307	110.4	936	219.3	626	121.6	217	39.9	58	11.8
10–14	2749	365.5	415	65.9	1183	161.4	794	94.8	280	33.1	77	10.3
15–19	3297	295.7	540	50.5	1536	129.8	865	77.6	282	28.5	74	9.1
20–24	3318	233.7	591	39.9	1534	99.7	835	62.1	296	24.2	62	7.8
25–29	2978	166.6	543	29.6	1273	68.4	797	45.2	299	18.3	66	5.1
30–34	2010	99.6	374	20.4	885	41.8	560	25.9	169	9.8	22	1.7
35–39	1053	49.5	282	13.4	506	22.3	215	10.4	48	3.0	2	0.2
⩾40	586	23.6	259	10.8	247	9.6	66	2.7	14	0.5	0	0.005
Mother's age at delivery (years)c,d
<20			2164	468.6	—	—	—	—	—	—	—	—
20–24			8574	1358.6	4229	772.7	713	138.3	—	—	—	—
25–29			6034	681.3	6648	836.8	2423	362.6	524	98.5	56	12.8
30–34			2065	170.5	3805	334.8	2786	292.2	838	110.0	149	24.6
35–39			747	43.3	1177	71.5	1248	93.9	689	60.2	179	22.0
⩾40			—	—	182	8.7	189	11.7	148	10.5	58	6.3

^aNumber of full-term births.

^bThe total number in first column is based on women with at least one birth.

^cThe numbers for age at Nth birth (N=1–5) are based on women with at least N births.

^dCategories marked with ‘—’ are combined with category below (older mothers) or above (younger mothers), respectively.