American Association for Cancer Research

Table 2.

Age at first birth and breast cancer risk stratified by hormone receptor expression

First author (reference)	Country (study)	Cases (n)	Age at first birth (y)			Risk estimate (95% CI)*
First author (reference)	Country (study)	Cases (n)				Risk estimate (95% CI)*	Reference group	Age
					ER+	ER−
McTiernan^† (39)	United States (CASH-WA)	240	<20	20-24	1.0 (0.5-1.9)	0.2 (0.1-5.0)
							25-29	1.5 (0.7-3.1)	1.0 (0.4-1.9)
							30+	2.6 (1.1-6.6)	0.8 (0.3-2.4)
Althuis^‡ (28)	United States (WISH)	1,375	<20	20-24	1.1 (0.8-1.6)	0.9 (0.6-1.3)
							25-29	1.4 (1.0-2.0)	0.9 (0.6-1.3)
							30+	2.0 (1.4-2.8)	1.1 (0.7-1.7)
							Nulliparous	1.6 (1.1-2.3)	1.1 (0.7-1.6)
Cooper (36)	Australia	380	<20	20-24	1.1 (0.5-2.1)	2.4 (1.0-5.9)
							25-29	1.3 (0.6-2.5)	1.2 (0.5-2.8)
							30+	1.6 (0.7-3.5)	1.3 (0.4-4.4)
							Nulliparous	1.4 (0.7-3.1)	1.7 (0.6-5.1)
Stanford^† (38)	United States (CASH-GA)	458	<20	20-28	1.2 (0.7-1.9)	1.0 (0.6-1.5)
Stanford^† (38)	United States (CASH-GA)	458	<20				29+	1.2 (0.5-2.8)	1.7 (0.8-3.7)
Kreiger (35)	Canada (ON)	528	<21	21-30	0.7 (0.5-1.1)	0.7 (0.4-1.4)
							31+	0.9 (0.5-1.6)	0.6 (0.2-1.3)
							Nulliparous	1.2 (0.7-2.0)	0.8 (0.3-1.7)
Wohlfahrt^† (22)	Denmark (DBCG)	6,044	20-24	12-19	1.0 (0.9-1.1)	1.0 (0.9-1.2)
							25-29	1.2 (1.1-1.3)	1.1 (1.0-1.2)
							30-34	1.3 (1.1-1.4)	1.3 (1.0-1.5)
							35+	1.6 (1.3-2.0)	0.9 (0.6-1.4)
Hislop (37)	Canada (BC)	512	<25	25-29	1.2	0.9
							30+	1.4	1.0
							Nulliparous	1.6 (1.3-2.1)^§	0.9 (0.6-1.4)^§
Yoo^† (41)	Japan	455	Continuous	Per 5 y older	1.2 (1.0-1.5)	1.1 (0.9-1.5)
Hildreth^† (42)	United States	148	Continuous	Per 10 y older	1.5 (0.9-2.4)	0.7 (0.3-1.4)
					PR+	PR−
Kreiger (35)	Canada (ON)	528	<21	21-30	0.7 (0.4-1.1)	0.8 (0.4-1.3)
							31+	0.9 (0.5-1.6)	0.6 (0.3-1.2)
							Nulliparous	1.3 (0.7-2.1)	0.8 (0.4-1.4)
Yoo^† (41)	Japan	455	Continuous	Per 5 y older	1.1 (0.9-1.4)	1.2 (1.0-1.5)
					ER+/PR+	ER−/PR−
Cotterchio^† (24)	Canada (ON)	3,276	<24	Premenopause
							24-27	1.2 (0.8-1.8)	0.9 (0.5-1.5)
							28+	1.1 (0.7-1.6)	1.0 (0.6-1.7)
							Nulliparous	1.8 (1.1-2.9)	0.9 (0.5-1.7)
							Postmenopause
							24-27	1.2 (1.0-1.5)	1.3 (1.0-1.8)
							28+	1.6 (1.3-2.1)	1.3 (0.9-1.9)
							Nulliparous	1.5 (1.1-2.0)	1.5 (1.0-2.3)
Britton^† (29)	United States (WISH)	1,212	<24.4	24.4+	1.2 (0.9-1.6)	1.0 (0.8-1.4)
McCredie^† (23)	Melbourne (Australia)	618	<25	25+	1.7 (1.1-2.5)	0.9 (0.6-1.4)
Huang^† (32)	United States (CBCS)	783	<26	26+	1.3 (0.9-1.8)	0.8 (0.5-1.3)
Huang^† (32)	United States (CBCS)	783	<26				Nulliparous	1.4 (0.9-2.2)	0.9 (0.6-1.5)
Potter^† (17)	United States (IWHS)	610	<30	30+	1.8 (1.2-2.6)	1.7 (0.8-3.9)
Yoo^† (41)	Japan	455	Continuous	Per 5 y older	1.2 (0.9-1.5)	1.2 (0.9-1.6)

First author (reference)	Country (study)	Cases (n)	Age at first birth (y)			Risk estimate (95% CI)*
First author (reference)	Country (study)	Cases (n)				Risk estimate (95% CI)*	Reference group	Age
					ER+	ER−
McTiernan^† (39)	United States (CASH-WA)	240	<20	20-24	1.0 (0.5-1.9)	0.2 (0.1-5.0)
							25-29	1.5 (0.7-3.1)	1.0 (0.4-1.9)
							30+	2.6 (1.1-6.6)	0.8 (0.3-2.4)
Althuis^‡ (28)	United States (WISH)	1,375	<20	20-24	1.1 (0.8-1.6)	0.9 (0.6-1.3)
							25-29	1.4 (1.0-2.0)	0.9 (0.6-1.3)
							30+	2.0 (1.4-2.8)	1.1 (0.7-1.7)
							Nulliparous	1.6 (1.1-2.3)	1.1 (0.7-1.6)
Cooper (36)	Australia	380	<20	20-24	1.1 (0.5-2.1)	2.4 (1.0-5.9)
							25-29	1.3 (0.6-2.5)	1.2 (0.5-2.8)
							30+	1.6 (0.7-3.5)	1.3 (0.4-4.4)
							Nulliparous	1.4 (0.7-3.1)	1.7 (0.6-5.1)
Stanford^† (38)	United States (CASH-GA)	458	<20	20-28	1.2 (0.7-1.9)	1.0 (0.6-1.5)
Stanford^† (38)	United States (CASH-GA)	458	<20				29+	1.2 (0.5-2.8)	1.7 (0.8-3.7)
Kreiger (35)	Canada (ON)	528	<21	21-30	0.7 (0.5-1.1)	0.7 (0.4-1.4)
							31+	0.9 (0.5-1.6)	0.6 (0.2-1.3)
							Nulliparous	1.2 (0.7-2.0)	0.8 (0.3-1.7)
Wohlfahrt^† (22)	Denmark (DBCG)	6,044	20-24	12-19	1.0 (0.9-1.1)	1.0 (0.9-1.2)
							25-29	1.2 (1.1-1.3)	1.1 (1.0-1.2)
							30-34	1.3 (1.1-1.4)	1.3 (1.0-1.5)
							35+	1.6 (1.3-2.0)	0.9 (0.6-1.4)
Hislop (37)	Canada (BC)	512	<25	25-29	1.2	0.9
							30+	1.4	1.0
							Nulliparous	1.6 (1.3-2.1)^§	0.9 (0.6-1.4)^§
Yoo^† (41)	Japan	455	Continuous	Per 5 y older	1.2 (1.0-1.5)	1.1 (0.9-1.5)
Hildreth^† (42)	United States	148	Continuous	Per 10 y older	1.5 (0.9-2.4)	0.7 (0.3-1.4)
					PR+	PR−
Kreiger (35)	Canada (ON)	528	<21	21-30	0.7 (0.4-1.1)	0.8 (0.4-1.3)
							31+	0.9 (0.5-1.6)	0.6 (0.3-1.2)
							Nulliparous	1.3 (0.7-2.1)	0.8 (0.4-1.4)
Yoo^† (41)	Japan	455	Continuous	Per 5 y older	1.1 (0.9-1.4)	1.2 (1.0-1.5)
					ER+/PR+	ER−/PR−
Cotterchio^† (24)	Canada (ON)	3,276	<24	Premenopause
							24-27	1.2 (0.8-1.8)	0.9 (0.5-1.5)
							28+	1.1 (0.7-1.6)	1.0 (0.6-1.7)
							Nulliparous	1.8 (1.1-2.9)	0.9 (0.5-1.7)
							Postmenopause
							24-27	1.2 (1.0-1.5)	1.3 (1.0-1.8)
							28+	1.6 (1.3-2.1)	1.3 (0.9-1.9)
							Nulliparous	1.5 (1.1-2.0)	1.5 (1.0-2.3)
Britton^† (29)	United States (WISH)	1,212	<24.4	24.4+	1.2 (0.9-1.6)	1.0 (0.8-1.4)
McCredie^† (23)	Melbourne (Australia)	618	<25	25+	1.7 (1.1-2.5)	0.9 (0.6-1.4)
Huang^† (32)	United States (CBCS)	783	<26	26+	1.3 (0.9-1.8)	0.8 (0.5-1.3)
Huang^† (32)	United States (CBCS)	783	<26				Nulliparous	1.4 (0.9-2.2)	0.9 (0.6-1.5)
Potter^† (17)	United States (IWHS)	610	<30	30+	1.8 (1.2-2.6)	1.7 (0.8-3.9)
Yoo^† (41)	Japan	455	Continuous	Per 5 y older	1.2 (0.9-1.5)	1.2 (0.9-1.6)

ORs were the reported risk estimates for case-control studies and RRs for cohort designs.

†

Adjusted for (a) number of full-term births/live births, (b) breast-feeding, or (c) both.

‡

Among premenopausal women ages 35-54 years.

Age-adjusted only.

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