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)*
Reference groupAge
ER+ER−
McTiernan (39)United States (CASH-WA)240<2020-241.0 (0.5-1.9)0.2 (0.1-5.0)
25-291.5 (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<2020-241.1 (0.8-1.6)0.9 (0.6-1.3)
25-291.4 (1.0-2.0)0.9 (0.6-1.3)
30+2.0 (1.4-2.8)1.1 (0.7-1.7)
Nulliparous1.6 (1.1-2.3)1.1 (0.7-1.6)
Cooper (36)Australia380<2020-241.1 (0.5-2.1)2.4 (1.0-5.9)
25-291.3 (0.6-2.5)1.2 (0.5-2.8)
30+1.6 (0.7-3.5)1.3 (0.4-4.4)
Nulliparous1.4 (0.7-3.1)1.7 (0.6-5.1)
Stanford (38)United States (CASH-GA)458<2020-281.2 (0.7-1.9)1.0 (0.6-1.5)
29+1.2 (0.5-2.8)1.7 (0.8-3.7)
Kreiger (35)Canada (ON)528<2121-300.7 (0.5-1.1)0.7 (0.4-1.4)
31+0.9 (0.5-1.6)0.6 (0.2-1.3)
Nulliparous1.2 (0.7-2.0)0.8 (0.3-1.7)
Wohlfahrt (22)Denmark (DBCG)6,04420-2412-191.0 (0.9-1.1)1.0 (0.9-1.2)
25-291.2 (1.1-1.3)1.1 (1.0-1.2)
30-341.3 (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<2525-291.20.9
30+1.41.0
Nulliparous1.6 (1.3-2.1)§0.9 (0.6-1.4)§
Yoo (41)Japan455ContinuousPer 5 y older1.2 (1.0-1.5)1.1 (0.9-1.5)
Hildreth (42)United States148ContinuousPer 10 y older1.5 (0.9-2.4)0.7 (0.3-1.4)
PR+PR−
Kreiger (35)Canada (ON)528<2121-300.7 (0.4-1.1)0.8 (0.4-1.3)
31+0.9 (0.5-1.6)0.6 (0.3-1.2)
Nulliparous1.3 (0.7-2.1)0.8 (0.4-1.4)
Yoo (41)Japan455ContinuousPer 5 y older1.1 (0.9-1.4)1.2 (1.0-1.5)
ER+/PR+ER−/PR−
Cotterchio (24)Canada (ON)3,276<24Premenopause
24-271.2 (0.8-1.8)0.9 (0.5-1.5)
28+1.1 (0.7-1.6)1.0 (0.6-1.7)
Nulliparous1.8 (1.1-2.9)0.9 (0.5-1.7)
Postmenopause
24-271.2 (1.0-1.5)1.3 (1.0-1.8)
28+1.6 (1.3-2.1)1.3 (0.9-1.9)
Nulliparous1.5 (1.1-2.0)1.5 (1.0-2.3)
Britton (29)United States (WISH)1,212<24.424.4+1.2 (0.9-1.6)1.0 (0.8-1.4)
McCredie (23)Melbourne (Australia)618<2525+1.7 (1.1-2.5)0.9 (0.6-1.4)
Huang (32)United States (CBCS)783<2626+1.3 (0.9-1.8)0.8 (0.5-1.3)
Nulliparous1.4 (0.9-2.2)0.9 (0.6-1.5)
Potter (17)United States (IWHS)610<3030+1.8 (1.2-2.6)1.7 (0.8-3.9)
Yoo (41)Japan455ContinuousPer 5 y older1.2 (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.